On the flipside, that also scares people away. New users want to take part and immediately hit a barrier. The place where everyone mingles is closed off. They have to learn why that is and how the Fediverse is supposed to work, find some instance overview list and make a choice. Be angry for a short while until they understand the concept and realize it's for the better... I think that'd be detrimental to the cause. I rather live with the issues that come with big instances than with a complicated onboarding process. But I think people already complained about onboarding on the Fediverse in general. I think we need to solve that issue first and then we can go ahead and also add some mechanism to steer people towards a more even distribution. But I don't see anyone working on any of that for Lemmy. Until then, I'd say don't do it.
(And btw: I don't want to see lemmy.world shrink, which wold be the outcome. What I'd like to see is other nice instances come into existence and grow to a similar size. Because they're a nice place and people can identify themselves with the community there. It'd foster good behaviour if things happened because of some good reasons. Not just you grow because you're already the biggest. That doesn't foster anything. It's just like playing Osmos.)