Yeah that's my big fear, that once Trump is out of office (one way or another) all the other nations of the world will just act like the trouble is over and go back to their old relationships with the US. I think that's a terrible mistake because, if the system in the US was healthy, Trump wouldn't have gotten anywhere near the presidency in the first place. And like you say, if he got in, someone just as bad or worse can get in too.
Once Trump is gone, the Heritage Foundation and Project 2025 will still exist. The dark money, Citizens United and Super PACs will still exist. Stephen Miller, Roger Stone, Elon Musk and Peter Thiel will still exist. I don't mean to minimize Trump or suggest he's just a harmless byproduct. He's a monster, but he's sitting on top of a very corrupt system that isn't going away when he does.