I disagree that just having large instances, in and of itself, is a problem.
the second part
Slowing growth is still a gigantic downside when growth is one of the most important needs for the platform.
For your scenario: You could argue that this is actually a good thing from your perspective. You realized there was a problem because lemmy.world is so big. If most instances were of equal size you likely wouldn’t have noticed there was a problem at all. I’m willing to bet there are other instances you have the same problem with and just haven’t noticed because of how much smaller they are - but lemmy.world’s size helps bring problems like this to light, so they can be fixed.
the third part
That would be spreading the power, rather than spreading the load, on a semantic note.
I don’t disagree with this section in principle - but I do still disagree that the solution is to close registrations. The admins have already stated they have plans to inform new users about other instances during the registration process, and soon. That’s a good faith effort and a good middle ground.
the last part
You having to create an account here isn’t because lemmy.world is too large - it’s because of software issues. You mentioned elsewhere that you made a post from your own instance about the problem (I assume in this community, otherwise why would you expect that to work?) - but if your problem was that content from your instance wasn’t showing up in lemmy.world, I’m not really sure why you expected that to work. It’s not disturbing that you had to create an account here, because you would have had to do so even in the hypothetical scenario where there are, say, 20 main instances with about 4.5% of the active userbase each.