I like ActivityPub and the decentralized social Web as much as the next person but we all know that scalability is a massive problem and I haven't seen too many convincing approaches to fix that. So let's not pretend this is just a magic silver bullet.
Blogs and the web didn't scale to actual web traffic (which admittedly many blogs never needed), centralized caches and services were necessary for this stuff to work. Which is exactly why we got so much centralization.
Solving that for decentralized social networks is the core issue.