I doubt that the vast majority of Twitter users made an active decision of giving Mastodon the thumb down. They just didn't hear of it, or had no impetus to find a different option. I think that's an important distinction.
That's maybe the sentiment of the users, but the implementation details allow indexing of public content very easily.
Anyway, the purpose - at least in my eyes - is not to compete with twitter as a whole, but to offer alternatives to people that desire it. And to this end, I think Mastodon, and I hope the rest of the fediverse proved quite successful.