It also isn't that Mastodon is necessarily a replacement for Twitter (or Lemmy for reddit). I use these communities vastly differently than I did those two places: I am much more involved and feel like an actual part of my curated communities. I'm not a lurker or a pair of potential eyes to be advertised at.
The ways that our Fediverse options are different are myriad and immensely important in a way that most outside observers don't get. These aren't "just replacements" at all; they're communities—and upgraded ones at that.