As soon as Lemmy instances are unsustainable out of pure interest for the concept of the Fediverse. I doubt there will be subscriptions, first it'll be donations, and then some instances may have ads. It's an inevitable that both will happen (either on the same instance, or some instances opting for donations to stay up, and others opting for ads to stay up). No one can run the servers necessary for this platform out of pure charity; the bill for the Fediverse is going to be due someday, and it has to be paid.
capitalism is when no removable batteries