First past the post doesn’t explain historically why we have only two parties, but it explains why we will continue to have only two going forward unless something changes.
Point (4) will likely strike many Americans as dubious, but the US doesn't have anything comparable to Québec – there is presently no US state where English isn't the majority language; nor anything comparable to Scotland or Northern Ireland (no state where a significant percentage of its population identify with a competing national identity). So, no matter how much (undeniable) diversity the US has, in the sense that counts for this purpose, it doesn't.
Hence: while abolishing first-past-the-post would no doubt do a lot to break the US hard two-party system, it could also be broken while retaining it if something happened to (2) or (3) or (4) instead.
But, in US politics, changing (2) or (3) is even more politically fringe than changing (1) is. Maybe that might change at some point though, who knows what the future holds.
And (4) will likely eventually cease to be true, but only in the long-run in which we are all already long-dead.
Changing that will require a radical reconsideration of democracy, like replacing FPtP with... something.
Scotland, Wales and Northern Ireland have nationalist movements that want out of the UK-state independence movements exist in the contemporary US too, but they are very fringe
But even England has not just Labor and the Tories, but also the Lib Dems, Reform, the Greens, Restore
So England proves you can have multiple parties with real power, with FPtP, even in the absence of regional-nationalist movements