I wonder if Fediverse, Blue Sky, etc will catch, or if it'll end up in the same boat. Threads too (yes it's backed by Meta, but G+ had Google behind it ...)
There would be pain if mastodon.social failed with zero notice. People would lose access to their accounts and would need to find a new server where they'd be starting over. Some may have backed up their contacts, but most wouldn't. If mastodon.social gave a couple months notice, people could migrate to other servers. given that mastodon.social is the largest server, there would be some growing pains as other servers worked to accommodate new users, but it's possible for the Fediverse to continue.
Note, I'm not saying that the Fediverse will be incredibly popular. I'm simply noting that there's an amount of resilience. Once Ello's owners ran out of interest or money, that was the end of Ello. Even if others had a huge interest in seeing it continue, there was nothing they could do. Even if the Fediverse doesn't "catch" by your definition of catching on, it has caught on for enough people who have moved there and will remain there.
That's why I feel happier in the Fediverse. It feels like something the community controls. Sure, I don't run my own server, but I possibly could in the future and there are enough people running servers that I don't feel beholden to any one entity. It just feels like something that can stick around - even if the cool kids aren't interested. Enough of us like it and we'll keep it going even if some of us become disinterested in it.
1. Meta's entire business is social apps, and Google's is not. There are strategic differences in approach as a result. 2. Google+ was an attempt to disrupt Facebook's rise at the height of Facebook's popularity. People _liked_ FB then - so trying to get them to switch to another product was harder. Threads shipped during a time of volatility with Xitter and is poised to capture more of that audience as Xitter continues to decay.
In terms of how things will change in the space over time, Threads choice to support ActivityPub will probably mean good things for the Fediverse in general, at least in the short term (3E notwithstanding), and could ultimately serve to be the arbiter that kills BlueSky and the AT Protocol.
Whereas threads has the obvious benefit of attempting to grow while Twitter is self destructing.
In contrast Meta isn't trying to Threadify everything. The addition of ActivityPub is an experiment that should be run in Threads.