You're sort of right, but with some very big and very serious caveats that kind of break your argument.

The first is that while 40tn is not a particularly troublesome number in a vacuum - it's about 125% of GDP, a little higher than France, a little lower than Italy (Japan is at well over 200%) - the issue is not the total number, but the rate of increase. Of that 40 trillion, half of that was accrued in the last 9 years. 10 trillion was added in just the last 3 years. So, much as with climate change, it's the rate of increase in the rate of increase that has people panicking.

Especially because this car is accelerating at an ever increasing pace right at the moment when the brakes are in dire need of servicing. US government spending has been cut to the bone, and yet they're still piling on debt at very rapidly accelerating rates. The only things left to cut are sacred cows. The only other option is raising taxes which no one has any political appetite for. So the people watching this rapidly growing debt pile are increasingly concerned that the US will not have the ability to service this debt. That erodes confidence, which raises borrowing rates, which further reduces the ability to service the debt in a vicious cycle.

Now, you're correct when you say that, in theory, the government can simply issue more dollars. But there are a few problems with that. The first is that the reason this debt exists is because the government does generally not pay its bills by issuing dollars. Instead it issues bonds, which private investors, institutions and nations buy. This is the process by which those groups lend the government money (you're familiar, I'm sure, but I'm laying out the details for anyone else following along). If those groups lose confidence that the US will repay those debts, they won't buy bonds (eg, won't lend), which can create a situation like what is happening in Russia right now where the government literally cannot raise money because their bond sales are failing.

Which leaves the government with its only option being to print money. Now while this is fine in theory (we'll get to the MMT stuff in a moment), there is an unavoidable risk; if you issue more dollars, you are inherently reducing the value of dollars already in circulation. At a small enough scale, this is fine. But if the US ends up in a position where it's needing to issue, say, a trillion new dollars every year just to service it's running costs and its existing debts, that can quickly spiral out of control. Yields spike, bond sales collapse, the government gets effectively shut out of the ability to borrow money and is left servicing its running costs purely on new currency printing.

Now, here we come to the MMT approach. For those not familiar, the horrifically simplified, absolutely missing a lot of critical nuance version of MMT (Modern Monetary Theory) is that you can basically look at taxation not as the means by which the government raises money, but rather the means by which it destroys it. This sounds weird but bear with me; every US dollar only exists because the US government prints it (usually via a major bank, but that's a rabbit hole). There are no other possible sources of US dollars. I can't just set up a rival press and make my own. So US dollars only exist because they're printed. When the government takes a dollar in tax, that dollar leaves circulation. It is, in effect, destroyed, because it might as well not exist until the government puts it back into circulation. For a government, the difference between spending a taxed dollar and spending a printed dollar is effectively nothing. Same goes for dollars raised via debt issuance; those dollars leave circulation until the government spends them, putting them back into circulation. Spending and printing are, effectively, analogous.

So MMT argues that the maximum possible supply of any currency is infinite. The purpose of taxation and debt issuance is just to remove money from the currently circulating supply when there's too much. That part is sort of easy to grok, but it comes with a really, really important caveat; when you over-supply anything, its perceived value goes down. So just because you can spend infinite dollars, doesn't mean you should. In practice, you have to balance dollars going into the system with dollars going out. And right now the US government is absolutely not doing that.

To put that another way, if you adhere to MMT then you can consider US government debt to be a useful proxy for the total supply of US dollars. Whenever the government takes on debt, that represents dollars flowing into the system (since they're not exactly sitting in government accounts; you take debt to service spending). So whether you are a traditional economist, or you subscribe to the MMT mindset of functionally infinite accounts, the problem being represented by that high debt figure - and moreso by the rate of its growth - is the same: The government is putting too many dollars into circulation, and not removing enough from circulation. This, sooner or later, tends to lead to rapid devaluation of those dollars. In more normal terms, that means inflation, and potentially hyper-inflation.