The Platform Formerly Known as Twitter will get forgotten. Threads will get some momentum and then people will forget about it. Mastodon will likely grow, but it’ll never grow to the same size as Twitter.
Reddit will also go kaboom. Most users will go to Discord, a sizeable minority will come to Lemmy, old style forums will see a small comeback. As the above, Lemmy will also grow, but not to the point that it becomes as huge as Reddit.
Depending on how the Kbin side of the fediverse plays its cards, it might go huge or it might get a small growth. It has the potential to attract users from both Twitter+Threats and Reddit.
Facebook will get worse. To the point that not even your grandma will want to browse it. Eventually Meta will decide to cut off the losses and divert resources to other enterprises (specially WhatsApp and Instagram), making it even worse. I don’t think that it’ll last 10y. I think that this model of social network won’t get a good replacement.
Instagram might get some losses, but nowhere as bad as FB. PixelFed will see a modest growth.
Discord will get huge regardless of enshittification, and this is what will promote the beginning of some activitypub alternative to it.
Some Wikia-like activitypub platform will pop up. Initially “attached” to other services (mostly Lemmy and Kbin), but eventually dwarfing them. The relationship will be mostly symbiotic though.
YouTube will stay. As shitty as possible. Google is not dumb, YT is one of its cash cows and they’ll tweak YT to be as annoying/advertising/shitty as still bearable by its users.
So overall I think that the fediverse will get huge, but none of the underlying platforms will. The main strength will be how they connect to each other.