They are providing a foundation that gets built upon with every migration wave, and I think it’s plausible that they will eventually break into the mainstream.
Put another way, the fediverse is the first alternative that doesn’t need to “succeed” in order for development to continue. It’s a bootstrapped model. And so it can grow quietly, work out the usability kinks over time, and be ready to absorb users whenever they get fed up with the centralized platforms.