Ethereum has hard forked once with the DAO. It needs a significant divergent of opinion - like 10% of all miners splitting off into their own network, client tooling, platforms, and ecosystem.
It is not something that can be done on a whim and can’t happen every time the US adds a new address to their sanctions list.
Technically, Ethereum has hard forked multiple times, for example to avoid "ice age" difficulty cliffs that were supposed to force the switch to proof-of-stake multiple times now. But the vast majority of miners, and importantly, the currency exchanges, have all been on the same side of those forks.
The contention was that it can happen to remove/censor contracts or transactions, not that forks don't happen more broadly. It seems the point still stands that there was a single fork around the DAO and the odds of anything like that again are basically zero.
It is not something that can be done on a whim and can’t happen every time the US adds a new address to their sanctions list.