My first comment apparently didn't go through so if that somehow materializes it's probably better than this one. But I where it went.

I think honestly a lot of what he said has fallen on deaf ears. Marx was an internationalist, deeply suspicious of the state and anti-classist. Most socialists today have fallen prey to the modern nationalistic mindset. They want to defend their glorious socialist state and bad mouth the evil bad capitalist ones so much they have no desire to be allies with the working class anywhere else, they want to praise their bourgeoisie and denounce the ones they don't like which is pretty much not what Marx was about. Modern socialists especially on lemmy here are more like national socialists, no pun intended, genuinely. Marx believed in the idea that a transitional government would form and become essentially obsolete. You might hear some socialists talk about this idea of a transitional government but that requires more suspicion and judgement on government than many of them are willing to do. The states continue to amass power all over the world and they often praise certain ones they have probably been conditioned to praise.

It's important to note Marx lived in basically a different world than today, he couldn't predict any of this. He's also not god, he isn't right about everything and often revised his own work. Socialists want to hang on his every word but as he said according to Engles "I myself am not a Marxist" in response to the growing movement in France.

Tl;dr - socialists today are more nationalistic and dogmatic than he would've ever liked to see.

Stalin's views were not at all at odds with Marx and Engels.

Regarding Critique of the Gotha Programme, it's important to actually understand the context:

Let us take, first of all, the words "proceeds of labor" in the sense of the product of labor; then the co-operative proceeds of labor are the total social product.

From this must now be deducted: First, cover for replacement of the means of production used up. Second, additional portion for expansion of production. Third, reserve or insurance funds to provide against accidents, dislocations caused by natural calamities, etc.

These deductions from the "undiminished" proceeds of labor are an economic necessity, and their magnitude is to be determined according to available means and forces, and partly by computation of probabilities, but they are in no way calculable by equity.

There remains the other part of the total product, intended to serve as means of consumption.

Before this is divided among the individuals, there has to be deducted again, from it: First, the general costs of administration not belonging to production. This part will, from the outset, be very considerably restricted in comparison with present-day society, and it diminishes in proportion as the new society develops. Second, that which is intended for the common satisfaction of needs, such as schools, health services, etc. From the outset, this part grows considerably in comparison with present-day society, and it grows in proportion as the new society develops. Third, funds for those unable to work, etc., in short, for what is included under so-called official poor relief today.

Only now do we come to the "distribution" which the program, under Lassallean influence, alone has in view in its narrow fashion – namely, to that part of the means of consumption which is divided among the individual producers of the co-operative society.

The "undiminished" proceeds of labor have already unnoticeably become converted into the "diminished" proceeds, although what the producer is deprived of in his capacity as a private individual benefits him directly or indirectly in his capacity as a member of society.

Just as the phrase of the "undiminished" proceeds of labor has disappeared, so now does the phrase of the "proceeds of labor" disappear altogether.

Within the co-operative society based on common ownership of the means of production, the producers do not exchange their products; just as little does the labor employed on the products appear here as the value of these products, as a material quality possessed by them, since now, in contrast to capitalist society, individual labor no longer exists in an indirect fashion but directly as a component part of total labor. The phrase "proceeds of labor", objectionable also today on account of its ambiguity, thus loses all meaning.

What we have to deal with here is a communist society, not as it has developed on its own foundations, but, on the contrary, just as it emerges from capitalist society; which is thus in every respect, economically, morally, and intellectually, still stamped with the birthmarks of the old society from whose womb it emerges. Accordingly, the individual producer receives back from society – after the deductions have been made – exactly what he gives to it. What he has given to it is his individual quantum of labor. For example, the social working day consists of the sum of the individual hours of work; the individual labor time of the individual producer is the part of the social working day contributed by him, his share in it. He receives a certificate from society that he has furnished such-and-such an amount of labor (after deducting his labor for the common funds); and with this certificate, he draws from the social stock of means of consumption as much as the same amount of labor cost. The same amount of labor which he has given to society in one form, he receives back in another.

Here, obviously, the same principle prevails as that which regulates the exchange of commodities, as far as this is exchange of equal values. Content and form are changed, because under the altered circumstances no one can give anything except his labor, and because, on the other hand, nothing can pass to the ownership of individuals, except individual means of consumption. But as far as the distribution of the latter among the individual producers is concerned, the same principle prevails as in the exchange of commodity equivalents: a given amount of labor in one form is exchanged for an equal amount of labor in another form.

Hence, equal right here is still in principle – bourgeois right, although principle and practice are no longer at loggerheads, while the exchange of equivalents in commodity exchange exists only on the average and not in the individual case.

In spite of this advance, this equal right is still constantly stigmatized by a bourgeois limitation. The right of the producers is proportional to the labor they supply; the equality consists in the fact that measurement is made with an equal standard, labor.

But one man is superior to another physically, or mentally, and supplies more labor in the same time, or can labor for a longer time; and labor, to serve as a measure, must be defined by its duration or intensity, otherwise it ceases to be a standard of measurement. This equal right is an unequal right for unequal labor. It recognizes no class differences, because everyone is only a worker like everyone else; but it tacitly recognizes unequal individual endowment, and thus productive capacity, as a natural privilege. It is, therefore, a right of inequality, in its content, like every right. Right, by its very nature, can consist only in the application of an equal standard; but unequal individuals (and they would not be different individuals if they were not unequal) are measurable only by an equal standard insofar as they are brought under an equal point of view, are taken from one definite side only – for instance, in the present case, are regarded only as workers and nothing more is seen in them, everything else being ignored. Further, one worker is married, another is not; one has more children than another, and so on and so forth. Thus, with an equal performance of labor, and hence an equal in the social consumption fund, one will in fact receive more than another, one will be richer than another, and so on. To avoid all these defects, right, instead of being equal, would have to be unequal.

But these defects are inevitable in the first phase of communist society as it is when it has just emerged after prolonged birth pangs from capitalist society. Right can never be higher than the economic structure of society and its cultural development conditioned thereby.

In a higher phase of communist society, after the enslaving subordination of the individual to the division of labor, and therewith also the antithesis between mental and physical labor, has vanished; after labor has become not only a means of life but life's prime want; after the productive forces have also increased with the all-around development of the individual, and all the springs of co-operative wealth flow more abundantly – only then can the narrow horizon of bourgeois right be crossed in its entirety and society inscribe on its banners: From each according to his ability, to each according to his needs!

Essentially, laborers cannot recieve the "full individual value" of their labor-power. What happens is that the proletariat becomes in possession as a class of the total social product, to distribute as a class. Between capitalism and communism is the radical transition from one into the next, known as socialism.

Regarding communism in one country, this is a matter of practical building of socialism. Peace benefits socialism, and socialist construction. Therefore, one should focus on highly developing socialism so that it does not fall under risk of falling. Socialism was established not in the developed capitalist world, but in backwards feudal societies. Socialists have had to deal with these implications post-Marx.

Communism cannot be achieved in one country, but the fact of the matter is that the correct course was to bunker down and pursue the socialist project regardless.

You cannot mechanically apply analysis from one situation to another. Marxists do not read Marx as evangelicals read the Bible, we read Marx to understand the world so as to change it. Marx was not a prophet, and thus the world developed in ways he could not predict. This is why he never bothered with mapping out communist society, because to do so would violate how we understand dialectical materialism.