Replacing one fallacy with another

Traffic matters. Usenet isn't actually dead (I still have a paid subscription to one of the handful of remaining text-only servers in the world), but the traffic volumes make it unreasonable for small sites to carry anything other than a curated feed (perhaps curated dynamically by subscription requests, I believe that's a thing now). I think that Usenet is significantly closer to the mastodon model of federated messaging than e-mail, and is probably a pretty good indication of some of the pressures involved. (How does mastodon deal with spam? Reputation scores or something equivalent to shared ban lists? Administrator-run spam-bots?)

The alternative to taking a cut-down feed (which is fractional de-federation, I suppose) is to have an account on an enormous server, where the feed volume is aggregated between the active users, and the message database is physically shared.

WhatsApp might just be XMPP, but it works because it's single node, an enormous SMP system has been competently engineered and refined to the limits of current technology. Cleaving that into federated pieces would necessarily complicate and slow down message delivery and introduce all sorts of synchronization and protocol issues. Seems like a reasonable design trade-off, if you can make it work. More secure than SMS and it's inter-exchange signalling issues.

The multi-dimensional discrepancies between network terms-of-service, national speech laws and population-group sensibilities is one of the more interesting struggles of our age, IMO.