The flipside of this is that the only people who actually bothered establishing an XMPP server at scale were those who wanted to spam Google or Facebook users.
Think of it this way: why doesn't Gmail just close their SMTP gateways and defederate from e-mail? Because there's shittons of systems that expect to be able to send mail this way. They'd have to spend loads of time and energy corralling people out of open standards and into a proprietary Gmail API for no externally-visible benefit.
That doesn't apply to XMPP. It was an entirely new protocol with only two very large adopters federating with one another. Legitimate automated systems that needed to send messages didn't really exist yet, so there was no legacy cruft holding people to the open standard. But there were plenty of spammers who realized that they could get into literally every Gmail and Facebook screen with it.
In e-mail, we have a complicated setup of blocklists, heuristics, and domain authentication to handle spam. This inherently costs more time and money to set up than just having a closed messaging system with sign-ups that are controlled by one entity. But the big e-mail providers deal with it because open[0] federation is an iron rule of the e-mail system.
[0] Ok it's more like "open if you spend enough time getting an originating IP that isn't on every blocklist, setting up SPF/DKIM, getting your recipients to add you to contacts and check the spam filter, and so on"
I could use any existing XMPP server, or even start one, and be able to contact directly with any of my both Google and Facebook contacts
It was wonderful glimpse of what it could be if greed and drive to herd consumers into their own walled gardens wasn't the main driving factors.