Random thoughts ...

If ActivityPub makes it into being a big deal in the future, I'd suspect some new stage of growth or development would be necessary. Either something gets added to the protocol or the architecture of the fediverse develops into a more sophisticated set of interacting services or the software ecosystem actually matures into an ecosystem. Without any of that I'd predict it stays relatively niche.

Should a decentralised protocol actually make it big, it could be game changing. But my "shot from the hip" bet right now is that ActivityPub and the current fediverse ecosystem is not it.

Apart from that, I'm actually thinking nothing terribly dramatic happens. There'll be some fracturing, but in the big picture it will remain relatively fringe with the core platforms remaining mostly in tact with large user bases.

If there's any real tectonic shift, I'd say that the 2010s idea of social media is on its way out (which is part of the reason why I'm not betting on the fedi going mainstream, as it's mostly stuck in the past). I think a big divide breaking now and into the short term future is that between private "true community" interactions such as in private group chats or on discord etc and public high-utility or high-entertainment content such as youtube, tiktok, wikis and maybe twitter going forward. Private chats will be where you have your network and the public domain will be where you extract value, with AI/algorithmic assistance playing an increasing role, or attempt to become a creator. Another reason why I'd bet against the fedi is that it tries to walk what I suspect is an awkward middle ground between private and public spaces without actually providing either.

How the great AI-ification affects things, I'm not sure. I'd bet it basically pushes social media into a winter of sorts, with the platforms that exist becoming more closed off (see Reddit API stuff) and the value of genuine human-only spaces going up (see private + public comments above).

Amongst all of that, I'd suspect that the platforms themselves won't really matter as much any more. You'll get whatever you're looking for wherever it's available from which ever service or creator is providing it, but it won't be a pleasant experience getting it and you'll feel generally bitter and frustrated by the experience. Meanwhile, you'll have whatever app(s) you need to stay in contact with those that actually matter to you, which again will depend on who's using what, not what you chose to use. Otherwise, everyone and everything you interact with will just be an ephemeral and confusing and increasingly detached internet blip.