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If everyone had to pay $10-20 per year, no there is no way this would be sustainable in a million years.

However, I am personally definitely willing to contribute $10-20 to fediverse communities/projects. If I have the spending money, I am willing to contribute way more, I believe in this place and understand the threat of not having free, diverse, at least semi-decentralized spaces.

I think making people pay will only ever work along the "to each according to their means" model, there are too many people from too many vastly different socio-economic situations to suggest some kind of blanket cost. Ok, so I can afford it but what about someone who doesn't live in the same country as me with entirely different wages and cost of living costs?

No, it doesn't work, but asking for donations with a suggested amount I think is a perfectly valid way to fund fediverse communities and I will absolutely contribute that much.