[Epistemic status: Uncertain, and also not American, so this is a 3rd party perspective]
As for the likelihood of some form of collapse, to me the current trajectory of polarization in the US seems unsustainable. Nowadays, members of both parties are split about whether they consider members of the other party “a threat to their way of life”(!) and feelings towards the other party are rapidly declining.
I do not think that this is just a fluke, as many political scientists argue that this is driven by an ideological sorting and a creation of a “mega-identity”, where race, education and political leanings now all align with each other. Political debate seems overwhelmingly likely to get more acrimonious when disagreement is not just about facts, but about your whole identity, and when you consider the other side to be your enemy.
It is only a slight overstatement to say that members of both parties live in two very different realities. There is almost no overlap in the trusted news organizations and the unprecedentedly constant approval rating of Donald Trump indicates that neither side changed their mind much in response to new information coming in.
On the up side, “67% comprise ‘the Exhausted Majority’, whose members share a sense of fatigue with our polarized national conversation, a willingness to be flexible in their political viewpoints, and a lack of voice in the national conversation.” My worry is that this majority is increasingly drowned out by the radical voices in traditional and social media.
It is also pertinent that political collapse can happen very fast and without much warning, like the Arab Spring and the collapse of the Soviet Union showed, which came unexpected to observers. Decline can also take the form of persistent riots/unrest where no one party has the political capital/strength to reach an agreement with the rioters or to stop it. Consequently, if decline of the US seems likely and bad, I would worry about it possibly happening quickly (<10 years).
Figuring out how to move politics towards the exhausted majority seems interesting. They probably care about stability a lot more than hyper-partisans do.