I was quite involved in XMPP, not from the very start, but quite early. At first its biggest strength were 'transports' – gateways to other, proprietary, instant messengers. Having a Jabber (that what it was called there) account allowed one to talk to ICQ and AIM users. This is what pulled first users and allowed the network to grow. The protocol being open and network being federated appealed to various nerds, for whom it became the IM network of choice. Especially when they could use it to talk to friends and family on other networks.
I wrote a Jabber transport for the most popular instant messaging platform in my country. It become a 'must have' component of any Jabber/XMPP server here. And some major local commercial internet services would start their own XMPP services – finally they had some means to compete with the monopolist. For me it was my '5 minutes of pride' – my little piece of open source software would be used by thousands of users, though most unaware of that. I have also wrote a Python library and a text client for XMPP.
Then Google joined and Facebook started considering it. It seemed like XMPP will become 'the SMTP of instant messaging' – the real standard which will end closed proprietary communicators. But things didn't go well. Google would often ignore the agreed protocol, change it a bit, while still declaring full support. XMPP development would slow down, as everybody wanted the protocol to be agreed with Google, but Google just made some small improvements on their side without sharing details or participating in building XMPP specifications.
Federation with Google would become more and more unreliable. Sometimes it would work, sometimes not. Google Talk, GMail Chat, Hangouts seemed to be the same thing and not the same thing at the same time it was a mess. Then Google pulled the plug. Then every smaller commercial providers did the same – there was no point in keeping the service when more than half of the contacts disappeared.
I felt betrayed by Google (it really felt like a 'non-evil' corporation back then). But that was not what killed XMPP for me.
I would have less and less people to talk to via XMPP, not just because of Google. Other networks my Jabber server was linked to become more and more irrelevant (anybody using ICQ, AIM or GG now?). Nerds that used XMPP left it because of loosing contacts in other networks, or just moved on to Discord (yeah… nobody seems to notice it is proprietary too). I would still use XMPP for family communication, but there was the spam…
Oh… the spam. I would get over hundred of messages (or contact requests), mostly in Russian, offering me bitcoins or cracked software. They would come from many different accounts and domains. Often from 'legitimate' XMPP servers. And there were no means to reliably block it. The XMPP protocol had no proper means to handle illegitimate traffic. XMPP servers and clients had little spam-fighting measures. The spam made XMPP unusable for me, so I shut down my server too. I guess that could also be a major reasons for some commercial services to de-federate. I think USENET was killed by spam and no effective moderation too back in the day.
Then my wife convinced me to bring it back. XMPP is again and still my primary communication platform for family chat. A private server with four accounts. Practically blocked from outside. We use it because it proven to be the most reliable thing and independent from the big corporations. Even Signal was inferior to that (no proper desktop/web clients, sometimes messages would be delayed even by hours, then it even stopped being convenient when they dropped SMS support).