You’re assuming that people act in their own interests. Conservatives will happily ruin their lives because an oligarch told them to. Just look at Brexit. Or…literally anything that happens in Republican states.

Yeah I think a breakup of the US would result in several successor nations. The 50 wouldn't remain independent for long. But that does illustrate the key difference between this and the case of the USSR. It's likely that such successors would have a much more equal share of population and economy than the post-Soviet case.

Though, another key difference is the history the US has for Constitutional conventions. Let's imagine the federal government goes away completely. By whatever means, 50 states are now independent nations. And as you note, after awhile most will get tired of being small tiny nations of little international clout. Now, one option would be the formation of new nations, perhaps formed from the regions of the old US. But there would also be folks running around calling for an entirely new Constitutional convention. Just a complete rewrite. And it is tempting to say that things are so contentious now that there's no chance the US could ever ratify a new Constitution. But, any Constitution would have to be adopted unanimously. This would be someone trying to get 50 independent states to merge. Each one individually could still say no. Which means that any new Constitution would be moderate and also structured with the kinds of grand bargains and compromises it took the original one to pass. If especially the new independent post-US states had some time to cool off, I can imagine after a few years a new movement to create a new Constitution would get going. There's already clear historical precedent for it.