This would incentivise instances to keep growing, as more users would potentially mean more income. I don’t think that this is a good idea as it conflicts the decentralized nature of the fediverse.
I would suggest that we promote donations more. Seriously, nobody is talking about that stuff.
Furthermore, “money” doesn’t always fix a communities infrastructure issues - people do. Most of the time, admins are a one-person-army. If people would take over work (like moderation, implementing helpful features in code, …) more often, that might help out the admins more than just throwing money at them.
It is the same as with a lot of open source communities: It’s not all about the money, it’s about people getting active and involved and helping each other out.