I think this is a false dilemma, Bernd.
I’d say that it would be great if those extremists would see using a distributed tool like Mastodon as the only remaining viable platform for them. It would not suppress their speech. But it woud deny them any amplification, which they now enjoy by being very visible on mainstream platforms, giving them the illusion they are indeed mainstream. It will be much easier to convince, if at all needed, instance moderators to not federate with instances of those guys, reducing them ever more to their own bubble. They can spew hate amongst themselves for eternity, but without amplification it won’t thrive. Jotted down some thoughts on this earlier in “What does Gab’s demise mean for federation?”