FOSS just does not have the aggressive scaling mindset. Even success stories like Linux' game compatibility and Chromium can be traced to just regular tech companies, as opposed to non profits.
Many non open source apps do get critical mass but they eventually go bust. Emacs, git, Linux and I think even Mastodon have a slower uptake but do not seem to have such a high risk of collapse. While YouTube and Facebook et al seem to have an insurmountable moat and collection of users the reality is recent history is littered with boom to bust failures:
MySpace, Vine, Yahoo all the way back to GeoCities.
I would be patient and only worry if mastodon is actively dying.
For me it's the only social media app I have installed.
I have both Mastondon and Bluesky accounts and in my experience I find Bluesky is just simpler to use which attracted more of the types of accounts I wanted to follow. Nothing aggressive about that, just good UX resulting in a richer pool of accounts.
