One of the best reasons to want ActivityPub (or similar software) to become the primary way that social media sites are populated with information is that it divorces the particular front end you use from the content that is displayed. Meaning that if, in the future, someone writes a new front end that is better/faster/whatever it doesn't have to (most likely fail to) fight the network effect to have enough content to be worth using. So you don't have a David vs Goliath situation for every new, innovative social media site to get off the ground. Never mind Mastodon or Lemmy or Misskey or Mbin. Maybe ten years down the line there are a host of newer and better fediverse sites that are usable right off the bat because they have the same content available that these current sites have. Look at what a trial it's been to get any new social media site off the ground (Bluesky included). It's in every user's interest to remove individual sites' ability to squash competition via the network effect.

Bluesky's model of decentralization does not allow for this so far as I know.