Maybe that's not a bad thing (from the perspective of Earth and other organisms). If we are gone, Earth would regain balance in few 10k to 100K years and that's nothing in geological time scale. Earlier the better I guess
But half the population has a particular vision for the nation, while the other half has a drastically different vision. As evident from the options itself, the OP thinks one generation’s vision is destructive, if that is their vision at all.
So people first need to agree on what ‘the united states’ stands, what its ethos is, and then you can evaluate threats.
A country is nothing more than an ideology, that majority people confirm to. That ideology can be the myth of a religion(theocracies like Pakistan, Saudi) the myth of a constitution(the USA, India) the myth of a shared history, or the myth of fear(china, russia)if that myth starts cracking, like it already has, according to some of the options in this very post, those cracks are the threat to established orders. So the biggest threat is not really any of the options that OP has written, but the very fact that he is keen to jott down these options, just to antagonize a few people and feel victorious in their agony
Why? Shared history is consciously or subconsciously used by many societies as a binding factor. The fact that a king is mythically respected and adored by two sets of people even though, as of today, they might be in different socio-economic classes, with one tottering in poverty and the other filthy rich, is testament to the shared belief in history. Happens in many ex-monarch countries. Belief in shared history a very powerful myth that can hold societies together.
Have to go with China. Mainly because they are funding campaigns to sway millennials to embrace socialism and when they buy into the propaganda those millennials in turn then encourage radical Islam & open borders. Along with all the other stupid commie BS they push.