Closing registrations will reduce the size because users are dynamic: New users join and old users leave with any system. Close registration and you’re left with only old users leaving.
I also disagree with the implicit argument that lemmy.world is “large enough”. It’s large compared to most other instances - but in terms of long-term stability I think the lemmyverse needs at least 10x the active user count it currently has and ideally much more than that. They don’t all have to join lemmy.world, but closing the registration page for the most popular onboarding point for the lemmyverse is going to slow growth no matter how you implement it.
Closing registrations to “spread the load” also comes with the assumption that server load from active users is a problem. By all accounts it is not a problem, at all, for lemmy.world. If a time comes where there are so many users that it is, maybe they’ll consider something like this.