Yes, they could, but it wouldn’t be the default and so far we’ve seen a strong norm in the community (and other blockchain communities!) to follow defaults. I’m 100% in favor of having the version removing the bomb be considered the minority/unofficial/etc. fork.
Well, if exchanges and other stakeholders do, they will given that most of the value will be on that chain. Miners are already aware of PoS, so I’m not convinced that a final upgrade would make much of a difference. Also, we could pick a “final” block to be one that would already be quite hard to mine due to the difficulty bomb, in order to make the choice to not upgrade less attractive.