Less resource-heavy than Matrix, doesn't have the "store everything from your every chat" feature and thus requiring less space, more mature, very easy to set up.

By firewall I mean outgoing.

I got what you meant. Anyway, if it's a company network, then they can, you know, allow something else.

Peer to peer is also a non starter.

That was in response to you asking how to do things without PKI, so I referred you to Retroshare as an example of using something like web of trust to that end.

P2P is irrelevant here. What does email have to do with this? Do you mean federation as in having servers, as opposed to distributed model? Do you mean identities being tied to servers?

And also why would that be "a non-starter"? Old Skype was P2P, using central servers for authentication only. I think we all agree it worked very well.

If you mean that it's hard - I agree, I love to blabber about P2P solutions, but these are harder.

(Say, since old Skype people got used to downloading their history on a new device, which didn't always work, but that can be solved by supernodes\servers to store and forward encrypted data with that history, a bit like Freenet. Only the person who can design something like that is definitely not me.)

What’s so good with XMPP?

What the other user said, plus having lots of good clients.

In general with XMPP thanks to the extension model (administrative one) good and bad things have already been tried, some discarded, and there's a specific set of XEPs making it a very usable protocol supported by all relevant clients.