Nobody can say it can't happen, but the reality is that the operators of the big instances so far at least have understood that there's an inherent damaging effect to defederating instances that has a sizeable proportion of genuine users that means it's generally treated as a last resort except for "purebred" abusive instances.

What happens if it should come to pass is that users will need to decide whether they like it like that, or move to a third party instance that doesn't block either one or only block the one they want blocked.

And the prospect of a user exodus (for a decision either way) is an incentive for admins to pay attention to what their users actually wants. To me that's just fine (though I'd like improvements to the user migration functionality to make this process even easier).

I could worry about this, or I can worry about Musk's whims. On Mastodon I can move elsewhere (and did; where I started was fine, but I'm now on my own instance because I like having more control). On Twitter, there's no recourse. As such I'll take those future hypotheticals over what's happening to Twitter any day.