>I'd put my money on something new taking Twitter's place way before Mastodon.

Something new, like what? When there's a big user exodus from some kind of service or platform, there has to be something for them to flock to right then. When Digg imploded, Reddit was there to absorb the exodus. Right now, Mastodon is there, even if it has significant downsides. Users aren't just going to wait around for something ideal to be built for them.