Proper mining pool decentralization should mitigate this to some extend, so it's important to work on.

Assuming the US holds a lot of Bitcoin by the time this happens and that ETFs allow people to cash out, what is the incentive for them to harm the network and crash the Bitcoin price?

I think maintaining the integrity of the social layer is equally important. Especially in states with a lot of hash power. The scenario you are suggesting implies that the people in government is not interested in holding Bitcoin on a personal level. I think ita becoming the opposite in the US?