I see only two ways for this to stop happening:

• Instance admins make a collective pact to close registrations if their instance reaches x% of the total active base, and only open again when this number goes down to x/2%. I think x=10 would be a good start.

• Instead of donations, instances start charging for access after a certain number. This is what my instance is doing: the first 250 active users are free, after that payment is required.