It depends; on the one side, the people 'inside' will probably become more radicalized, but on the other they no longer have a platform, new members of the 'club' are harder to find, and they no longer cause stress / strain on (in this case) Reddit. Reddit must've had hundreds of complaints daily about something posted in T_D. I know I complained once because they adjusted their algorithms so that T_D wouldn't be as prominent in the top lists anymore, but given that I'm one of those idle scrollers on r/all, eventually I went down the list so far that EVERY post was a T_D post.
The only reason to go there is if you are already of that mindset. Without access to the wider reddit platform they might end up having a harder time recruiting new users.
I think the fact that Reddit and similar sites block URLs to competitors like thedonald.win is a really good argument for an anti-competitive lawsuit. They are overtly suppressing competition.
