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A reminder in the age of the Fediverse: if you're not the customer, you're still the product.
(slrpnk.net)
OP was taking about Tumblr, but I think it applies even more to the Fediverse: users need to develop an ethos of paying to support the sites they use. Otherwise advertisers pay the bills and call the shots.
The server costs can be quite modest. The one big exception is streaming video. If you're willing to give that up, a bunch of Raspberry Pis will get you pretty far. Even further by bumping up to old desktops running Linux.
I've had a project in the back of my head about laying out what a Solarpunk Internet might look like. If you're willing to drop streaming video (or even just drop down to 480p), that version of the Internet becomes a lot easier to setup on very modest hardware that's otherwise considered e-waste.