I believe that when Gab switched to Mastodon, the integrity of the Fediverse as a whole was questioned. Sure there was instance blocking and these same conversations beforehand, but Gab brought this to every admin's front door.
I have been thinking that @freemo's idea of the group of instances is the way to go, or it should at least be tested. Groups of instances which will not block each other could improve the situation somewhat. There needs to be repercussions to blocking everyone with whom you disagree.