First of all we had big instances die. Like feddit.de and kbin.social That always damages a big part of the network. If things were distributed more evenly, it'd be a smaller chunk of the fediverse that vanishes in such a case.

Then, being way bigger than the others gives someone disproportionately bigger power. If you're not having any issue with that, you might as well join Reddit. And the first big Lemmy instance (lemmy.ml) arguably does that, too. They'll act against you once you say something negative about communism, China, ... and that's not okay. Now we have lemmy.world as the bigges instance and it's way better. But still I've also read people complain about their moderation practices.

If we have some dominating entities, they'll disproportionately shape the tone, atmosphere and behaviour on the whole network. We might or might not want that.

In the end I think what actually happens should reflect the vision and the capabilities of the software. The Fediverse is supposed to be an interconnected network of instances. If the technology works as intended (and the vision behind the Fediverse is correct) I expect that to manifest in the way it actually grows. If it favors one or two large instances, we either might have an issue with the technology/software and it's not able to truly achieve it because of some shortcomings. Or the idea behind all of it might not be more a theoretical concept than viable in the real world.