One big difference I see, between email and various messengers on the on side and social networks on the other side is, that emails are me pushing my information at people while a social network is supposed to be them pulling information from my feed. This allows me—when it works—to share stuff with people instead of potentially sending spam. Not all communication is a digital postcard. Social networks allowed a new form of communication. I enjoyed OG facebook until the first big privacy scandal. It was a (mostly) good thing and it was very accessible even then. I‘ve never got such a large part of my tribe on a pull service since then. Sure we all have messengers and groups and what not… but it is not the same. I dream of self-hosting something from the fediverse but we‘re not exactly young anymore and I doubt my tribe will assemble.

It‘s not just about the data—which is bad enough but as you said they could just write a crawler to get at it. The question is why would they want to federate and why now? Meta being Meta the most likely reasons are terrible for the fediverse and it reminds me very much of Google and xmpp. I saw a really good writeup on this yesterday: https://ploum.net/2023-06-23-how-to-kill-decentralised-networks.html