Yes, the first wave of Fediverse adoption came from people looking for a Twitter replacement.
But the next wave of adoption is going to come from an ecosystem of apps.
Why?
Because some of those Twitter migrants were developers.
And once they played around with the code, they all remembered how nice it is to build something on an open protocol.
Why are developers supporting the Fediverse even though the Fediverse "only" has 10 million accounts?
Because the Fediverse offers something developers crave: stability.
As Twitter already demonstrated, they can remove API access from developers for many bullshit reasons -- with no explanation.
No company should ever depend on Twitter's API -- or any Big Social API for that matter.
But ActivityPub is a W3C-backed web standard. It is an open protocol. Unlike Twitter, it is more trustworthy.