Why is JSON better than XML? It's more modern, sure, but from technical perspective it is not objectively better right? Not something worth switching protocols for.
XML is unnecessarily complicated. By trying to cram everything into the spec, it's cumbersome and hard to parse.
You mention XMPP has transports as opposed to Matrix bridges. I thought they give you roughly the same outcome. What's the difference?
The goal is the same, but the way they archive that is different. For transport to work, you need an account on each platform you are using the transport on. It relays the messages through that account by mimicking the client. While bridges work by relaying the messages between rooms and not specific users.
My understanding is limited, so if you are interested, please do your own research.