Canada

What you've said is true, but I still insist that Canada is not an imperial or opportunist country that is looking to gobble up states, even if handed on a silver platter. It's just not really what we do. You are severely overlooking the fact that it would require a supermajority of support in both the seceding state and Canada as a whole to admit a new province/territory into the confederation. That is simply unlikely.

If the US collapses USSR-style, it would likely just simply balkanize instead of being picked over by its neighbours. It would balkanize either by region, or completely by states. What is probably more likely is probably another civil war, with one side completely winning or a stalemate being met and two or more separate USAs existing. But honestly, there is literally no way to know what will happen to the US going forward.

Gun to my head, had to choose, I'd say that America just continues to be a flawed democracy and returns to its isolationism that it historically always does inbetween major wars, probably suffer a major recession, and then has a civil war.