I also remember when XMPP wa still the cool kid on the block. Gtalk and other chat networks supported it and allowed federation. I tan my own XMPP server and could talk to users on other servers and even networks. But then Google cut the federation and eventually all external access.
It could have been the next email, but big corporations were already in the chat space and they all walked in their user base.
I’m fairly certain that if email (SMTP) hadn’t been the dominant protocol, we would have walled gardens there as well.