As much as I want to see Mastodon succeed, I don't see it ever happening. Its biggest feature (multiple instances) is also the biggest reason most users are never going to use it. It's just too confusing for the end user. Also it's bizarre that the one instance that actually has the potential to gain mainstream interest (mastodon.social) is not accepting new invites.
I'd put my money on something new taking Twitter's place way before Mastodon.