An open garden cannot compete with Facebook, Twitter or Spotify. We need regulation and legislation which clearly outlines what is and is not okay, while creating black box behaviours that is impossible to navigate.
Maybe not for your exact use-case. But "open" doesn't need large numbers, market saturation or a monopoly: those are traits that come from investor-fueled business-models.
A mastodon account that has great communication with seven other accounts is a 100% success! It doesn't need access to billions of people, it just needs to fulfill a need: talk to several fine people. And it often does this just fine with a very small social graph, even.
A musician doesn't need a million streams, they need enough revenue to be an income. On spotify (or youtube, etc.) that, indeed, means "millions of streams". But it could just as well be five tshirts sold, twenty-eight .zip files @ €9.88 sold, and three vinyl shipped in a month.
The "numbers" that mark "competing with" faang, really don't matter. We don't need to make billions, we just need to make a good income.
Mastodon is not an “open garden” and can easily, if not more easily, be as arbitrarily administered as any of the social media platforms you’ve mentioned. Each servers admin can and have cut off access to other nodes.
See what’s happening with journa.host as an example.
If it has to be "better" then it must improve. But improvement doesn't mean "competing with FAANG". It just means being good enough today, and keep improving from there.
My income 35 years ago was vastly different from today. I needed far less, but it also grew over time. But that doesn't mean I must make millions today to consider my life a success. I'm happy today. Isn't that "successful"?