If ActivityPub succeeds, it's unlikely that the Mastodon server will be the only mainstream implementation.
Mastodon is already not the only mainstream implementation. It wasn't even the first mainstream implementation. https://fediverse.party/ lists ten.
It's been a looooong time since I mostly did web dev, but if anything got me back in the game, this is it. I'm sad to see Twitter implode, but this unnatural disaster is a window of opportunity that feels like the mid-2000s social media Cambrian explosion all over again.