I'm not a lemmy.world user (so I have no dog in this debate) but I'm gonna disagree. I'm of the opinion that signups for an instance should stay open as long as the admins can afford/handle the growth. I think it's unfair to force the admins to keep it open if they aren't able to handle it which is the crux of what OP is asking.

Besides, it's not like those users cant just federate with all the communities of lemmy.world they want. And I bet there won't be a dearth of choices for decent instances if the demand calls for it. Lemmy.ml closed signups despite being the big instance and lemmy.world emerged as an alternative. So why wouldn't something similar happen if lemmy.world did the same?