IMHO, the big problem is that defederating causes a bunch of problems when you’re cutting ties with an instance that is 99.9% good actors.
Sure, you cut out the trolls, but you also totally fuck up the user experience for the vast majority of well intentioned people.
Example, I’m on Lemmy.world, and my wife is on beehaw. We often both comment to a Lemmy.ml community, and neither of us can see each others comments even though Lemmy.ml is neutral ground.