My (maybe naive) understanding is that proof of stake is actually better here, no? Stakers would need to commit to one particular fork and accept that their stake (in ETH) would be lost/slashed in all other forks.

In contrast, in PoW, there's nothing stopping miners from creating a hostile fork that is economically advantageous to them (despite maybe being "worse" overall) and then flipping back to the original chain if their fork doesn't work out. Their hash power can't be slashed in the same way stakers ETH can be in PoS.