I think that is always going to be a fundamental issue. There will always be instances that are too large in the "founding federalist" eyes. People are always going to flock to the place they see other people, or where their friends are. The cat is kinda out of the bag as far as that goes.
Blaming this instance seems to be missing the forest for the trees. Having a bunch of instances semi equally sharing all the users would be great, but I don't think it's ever going to happen. Humans like to be in groups, there will always be a "winner". I think the question is how to mitigate fallout when there's a failure.
It's probably a bad idea for me to reply, but this doesn't read well coming from an admin. For someone like me who doesn't know shit this reads like you know there are no issues with the backend of lemmy and this instance just selfishly grabbed up all the users and won't let them go anywhere.
If 10 people decided to do the same thing to 10 different instances would they all suffer the same downtime lemmy.world is? If so that does sound like a lemmy issue, not an instance issue.
I get not wanting any one instance to get too big, and that it goes against the core thoughts of federation, but shit happens. All that can be done is informing people as they onboard.
If I'm way off base let me know.