> IIUC Jabber never really had decentralized group chats?
Jaber had something like that 5 years before Matrix: https://xmpp.org/extensions/xep-0289.html but decided it wasn't an avenue worth pursuing, as a non-issue in practice. This is further proven by WhatsApp (which is based on XMPP/ejabberd) and its billion active users: you don't have to shove this complexity into the protocol level when it runs natively on a distributed/federated infrastructure (the erlang VM in this case).
> Matrix rooms are fully decentralized and can survive any servers failing.
In practice the room survives in a "split brain" mode where different users see different participants, the room isn't available to the users whose instance went dark as its no longer hosting their identity, and messages are popping mid history while the state resolution is working hard to reach consensus. When the servers finally come back online, there is a risk of never catching up fully, or worse, for the rooms to remain split forever.
And I adventure a hot take there: this property of staying seemingly online (in a frankenbizarre way) came handy way fewer times in the history of Matrix than for all the incurred complexity reflected in daily bugs, privacy, security, performance issues and inconsistencies.