I think Mastodon will continue to be expensive to host, as the way it's engineered is not well suited to cut costs, but I also think we'll see a lot of innovation in "slimmer" services that will be "Mastodon-compatible enough" but lighter weight. Ideally we'll eventually see some sort of standardisation of a client-server API (rather than just depending on people ad-hoc reimplementing Mastodons API), but that may take longer.