Lemmy and other federated solutions will get a big boost in users, but it will only very be a tiny fraction of the reddit userbase.
And 98% of those users will probably just head back to reddit in a week or two.
Subreddits that have closed and moved with be replaced with new subs on reddit.

I think in the end it will be a healthy boost for Lemmy, but so far I suspect don't think we are at "Mass Paradigm shift" yet.

This is not going to be Digg > Reddit

Lemmy has Blackjack???

I am a shitty programmer, and I would never want most of what I do to go anywhere near Main.
But as you said, I have a few things that I run as forks, with my own little tweaks.

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