My main gripe with ActivityPub is that the infrastructure basically replicates 1-to-1 across subscribed instances. It means that as lemmy grows, servers will require more and more storage to keep up. For now, it’s fine since we’re under a few TB of content on the platform.
If lemmy were to be as popular as reddit, we’d reach the dozens if not hundreds of TB of storage required. Not everyone has the money to build such a homelab or rent data center servers of that caliber.
ActivityPub in it’s current state is nothing but replicated centralization, not a full decentralized protocol. We’d probably need a different database system that handles cross region clustering and sharing to scale it up.