Seems so simple that somebody would have tried it already
Today you are one of the lucky 10 thousand: https://en.m.wikipedia.org/wiki/App.net
cost of running a popular site.
I know for a fact that I can run an instance with 15k users and if each one paid $10/year I could make enough to make a living, hire someone to help with moderation and would let me have time to contribute back to the codebase and work on more fediverse projects.
The beauty of this is that I don't need to have a "huge" site or a monopoly in the market. Other developers could do something similar, due to federation there could be space even for collaboration and/or expansion into other segments.
All we need is to get more people to understand that paying $10/year for something they used to have "for free" is a lot better than having your data exploited.
In terms of costs, the predominant factor is storage, which does not go away and is ever increasing. But anyway the problem of instances with thousands of users is not the cost of hardware, but the labor involved with moderation, security, support...