Trump wouldn't have sent arms like Javelins to Ukraine in 2018 and he would have stopped messing around in Syria.
Putin had no reason to stop Trump from doing that.
Remember, until February 2022 Putin knew that Ukraine will become a part of the Russia in 2022. He had a lot of intel from actual Ukrainians living in Ukraine. He had spent billions for bribing various Ukrainian military officials. He knew that those Javelins will all be in the inventory of the Russian armed forces in spring 2022. Why would Putin have forbidden Trump from sending them?
Of course, Putin had not understood how much 2014 changed the Ukrainian thinking. The Russia was still more or less a friend until that. After it attacked Crimea, it absolutely was not. And suddenly very very few people would coöperate with the Russia even to the least extent. It's difficult to explain how abrupt the change was! It was not possible for Putin to understand what had actually taken place, because the Russian heart is so very different from the Ukrainian heart. Russians would never have had a Maidan. It would absolutely not fit the Russian culture to do anything like that. The feelings behind that were simply unfathomable for a member of the nation Putin hails from. But in any country there are a few percents of the population that really wants to see shit where there is none. And those felt uncomfortable in Ukraine. And those people really did think their country was going to hell and that's what they told Russians. That was really how for example Medvechuk saw things. While extremely unrepresentative of Ukrainians, those people were practically 100 % of those who gave any intel to the Russia.
And of course...
Putin rules through corruption. He gives people power by electing that for certain people no corruption will ever be noticed by the authorities. And at the same, he has the power to make the authorities see the corruption whenever Putin needs to get rid of those people. But if you choose only public officials who are corrupt, of course they will steal all the billions they were supposed to use in Ukraine. Especially since Putin told everyone around him, even Shoigu, that there will not be an attack against Ukraine – that everything is just training for the worst of the worst case scenarios. Preparation in case that EU goes crazy and attacks. And everybody knew EU won't attack and they trusted Putin when he said he has absolutely no plans to attack anything in the western direction.
But yeah, Putin has done his homework. He had made a lot of effort to get it right. And therefore he knew it doesn't matter what weapons Ukraine has. He "knew" that Ukrainians overwhelmingly want to join the Russia and he "knew" that all military districts in Ukraine had been bought with those billions he had spewed there. The more weapons Ukraine gets in 2018, the more weapons the Russia has in 2022 when it's time for the next step of the plan – the Suwałki gap, next to which there had been several very very big wargames when the future events of 2022 seemed crystal clear to Putin.
It's just... Sometimes you put a huge effort into your homework and still get a wildly incorrect answer, if you have unknowingly slept through the most crucial lessons.
I think the next guy has a high likelihood to recognize that the war cannot be won. Putin probably knows the same by now, but he has no other options than to continue (unless he's okay with dying, but he isn't).
A successor has the option to declare something akin to a coup (even if it really isn't one) and claim that "it was Putin, not the new Russia I am leading!", getting the soldiers out of Ukraine and starting the negotiations. (Or rather: starting negotiations for withdrawing Russian troops from Ukraine and drafting a real peace deal.)