Just like with email, I think the future of the Fediverse will involve institutions and companies running their own instances for discussion related to their niche.
For example, universities might run their own servers for campus-related discussion, and game companies (Paradox Interactive comes to mind) might run a server for discussion around their games and by their members.
Running a server is expensive, and in the long run I think the sustainable future will be for established institutions with large budgets to put a tiny part of that forward for instance hosting, rather than individuals self-hosting instances that actually lose money even when buffered by user donations.