I've never run a server, so I can't really say much about how sustainable it is to do it right now, but ultimately I don't see why it should be able less sustainable than running any other popular website.

Granted, I think you're totally right that there's a generally unsustainable attitude that's pervasive on the fediverse and the open source community in general, which amounts to a sentiment that "someone else will pay for all this". It's wrong, it's naive, it's unhelpful, and it's basically an express lane towards the tragedy of the commons. I've worked for non-profits and I've seen first hand how difficult it can be to turn users into supporters, but the sad truth is that non-profits are just like businesses in the sense that if costs are higher than revenue they will not survive very long, and this is true for community run fediverse services too.

I do think that people who like the fediverse should want it to become financially sustainable, at the very least.

I'm open to the idea of limited, non-invasive ads for example. (Plus I think that if the fediverse ever becomes massively popular we're going to see thinly veiled ads anyway, in the form of "influencers" and "sponsored content". That's inevitable, and honestly probably even worse that straight-forward ads.) I would not leave my Kbin.social or my current Mastodon instance if there were a small number of ads.

Also I could be wrong on this but IIRC, Misskey supports user data storage quotas that can be expanded for a price. And I think that's potentially a smart and sustainable method of getting those people who make heavy use of their server to chip in a little bit. If someone wants to post a lot of images, audio and video to their Mastodon, Pixelfed, Peertube, Lemmy, etc., instance then I think it's reasonable to expect them to cover some small fraction of the hosting cost by becoming a paying member or paying for a server-level storage plan.