Mastodon just doubled its active user count to around one million; it's already benefited. It doesn't need to replace Twitter to succeed, just provide value to its userbase.
Mastodon instances have certainly gained tons of new users...but, wait 'til some people learn that the fediverse has been around since ~2008, and the current numbers are far higher than *only "mastodon"*. For example, i'm seeing over 6 million accounts as captured before end of October: https://fediverse.party/en/fediverse/
Of course, on the fediverse, no one cares about adoption numbers. :-)
I think it’d be better to judge it once those users have had a little time to try it. I signed up but couldn’t find anyone to follow, found the lack discoverability baffling, and ultimately bounced off.