It's not going to grow much.
95% of redditors won't make the shift.
The Startrek community on Reddit is just going to start spinning up new subreddits to replace the ones that were closed.
Sure, it will take some time for the new communities to reestablish but it will happen.
I support and regularly use federated systems and support the principals of the Startrek subs that have moved, but the reality is that societally/practically we are a long way from people choosing to move away from the various MegaSocials.
I suspect people being people there will be a larger surge to the Fediverse once third party apps start failing.
Just to be clear, I have no objections to the move, and agree with the stance, I just have doubts about people leaving their places of convenience.
In general I find the kind of person that moves to the fediverse to more interesting anyway.
Wishing this community success