You're thinking too narrowly about the types of "hard forks" that are possible and what the space of all possible regulations could be. For example, one possible idea (with a lot of downsides! this is just an example, not a proposal), is that the US government could just promulgate a "US super-key" that allowed it to sign any transaction and have it be considered valid, and require users running blockchain software in relation to financial applications to respect those transactions. This would be a bad proposal for a number of reasons, but it's
possible, because blockchains and the code that enforce them are inherently a social construct, an agreement made between all participants.
But the answer to "why would I want to to transact on a chain that is under constant threat to be forked off" is even simpler: It's because, in this hypothetical, the regulatory environment you operate in gives you no other choice. Unless you and everybody you transact with has the ability to boycott or subvert the regular financial system entirely (e.g. you're doing entirely black market transactions), then you'd have to fall in line if a government that was crucial to your operations or your downstream supplier's operations required it.