The problem with the "Let's join the US!" idea is that it assumes a large majority of American politicians would support the idea, which I find questionable. Certainly some percentage would decide it was a bad idea because Canada is too left wing, and others would likely join them when they looked at Canada and decided it was too left wing. It also seems likely that whoever was in government at the time union might occur would worry about the effect of millions of new voters. Especially since some significant fraction of said voters might be pissed off at annexation, and vote in unpredictable ways as a result. Then there'd be things like state governors not wanting even more competition for federal money, ant the pesky problem of Puerto Rican statehood.