Why would Nazis (or flat-earthers or QAnon or any other group likely to receive _substantial_ challenges to their world-view) stay somewhere where they'd be challenged, though? On a Reddit-like platform they'd downvote and/or ban to ensure it's almost entirely an echo-chamber, like every other part of Reddit.
If they somehow weren't able to control that, then I suspect they'd move to another platform organically, because why would they put up with a constant flow of opposing viewpoints that these groups believe, with absolute certainty, to be wrong?
They would stay there because there's a huge userbase to convert and they probably enjoy being challenged and justifying their horrible views because they want lurkers to read it.
If you ever meet an actual Nazi you should ask them if they want to discuss Jews with you. I'm willing to bet they won't shy away from accepting the invitation even if you were offering it because you wanted to challenge their views.
It takes a very special kind of person to openly be a Nazi and they don't think like your typical Joe.
If they somehow weren't able to control that, then I suspect they'd move to another platform organically, because why would they put up with a constant flow of opposing viewpoints that these groups believe, with absolute certainty, to be wrong?