That's not how it works. It's open source and can be forked and improved upon. It doesn't need millions of users to survive and be useful to a lot of us.
Outside of the tech circle, virtually no-one cares at all after 6 years of existence especially with the constant comparisons to Twitter back then and today. This also explains that many people who left Twitter to Mastodon months ago came right back to post on Twitter again because the level of interaction on Mastodon is extremely low.
It isn't early days anymore and 6% of registered users are not using it. If that is not a failure to convince Twitter users to use or migrate to Mastodon, then I don't know what is. Not exactly a convincing elevator pitch for normal users to use outside of tech.