I see the fediverse growing slowly over time as people realize that centralized services are fundamentally doomed in several ways. It's always only a matter of time before a bad UX change, or they're acquired and they sell your data, or the owner turns out to be a sex offender, or the platform is taken over by trolls, etc. Each time one of these mass exodus' happen, most users will jump onto the next doomed bandwagon, but some small portion will try out the fediverse where they'll realize they're now relatively immune from these fatal flaws. An instance might go down, but the rest of them will move on without them.