Honestly, Reddit is likely to keep on trucking with a decent sized user base no matter what. A massive number of people aren't gonna leave, if for nothing but simply not wanting to have to change. I think the most likely thing that happens is that Reddit loses a small chunk of people, their growth heavily slows due to competition and a slow trickle of people leaving (but likely offset by the network effect still favouring them for new people), and they take a revenue ding because advertisers aren't gonna like all this drama.

The Fediverse will probably have a bit more rapid growth as the blackouts still continue in some subs and more people become aware of alternatives to Reddit, but then just grows slowly, with usability being the big barrier to massive adoption.