I think my belief that donation-supported instances won't scale comes from the assumption that the users donating today are those that do so for ideological reasons, they want to see the Fediverse succeed, they are anti-capitalists etc. Most of these type of users are already on the Fediverse, as you move towards "the average user" that propensity to donating gets rarer and rarer, because they just want a social media platform that works and are perfectly fine with ad-supported models of alternatives, so I assume that percentage of users willing to donate does not stay steady with growth.

But a good example of a project that has managed to get even the average person to donate is Wikipedia, so maybe with enough nag-bars and the appropriate messaging Fedi instances will manage to do so as well. I certainly hope so! I also hope to see other non-commercial entities like not-for-profit institutions and government bodies on the Fediverse but again I believe these tend to move slowly and only adopt things that have sufficient momentum, momentum that might come from the Meta move.

It will take longer if no big company very involved but I don’t think we are in a hurry here.

In my opinion there is some hurry, we've already seen Mastodon user count slumping before the latest Twitter fiasco and alternatives like Bluesky and Threads are coming online, whether they federate or not. Social media relies on network effects, and the current collapse of Twitter is a golden opportunity for the Fediverse to get that critical mass necessary for widespread adoption. Slow steady growth might not be possible, as people don't tend to stick around if most of their (para-)social circle is consolidating on another platform.