What AT enables is forking products with their data and users. So, if some product is going down a bad road, a motivated team can fork it with existing content, and you can just start using the new thing while staying interoperable with the old thing. I think this makes the landscape a lot more competitive. I wrote about this in detail in https://overreacted.io/open-social/#closed-social which is another longread but specifically gets into this problem.
I hear you re: not wanting "weird aggregation", that just be a matter of taste. I kind of feel like if I'm posting something on the internet, I might as well have it on the open web as aggregatable by other apps.