Same thing that happened with Google Talk and XMPP.

https://ploum.net/2023-06-23-how-to-kill-decentralised-networks.html

Google Talk adopted XMPP, and was compatible with other servers running XMPP. Google's adoption led to many new people starting to use XMPP through their service, and people using XMPP on other servers could now talk to their friends on Google Talk.

XMPP then attempted to create new features that would be adopted by every XMPP server, except for Google Talk. This led to incompatibilities, where XMPP users wouldn't be able to talk to Google Talk users for days at a time. The large number of Google users meant that XMPP couldn't just ignore Google users, and would instead have to slow progress to remain compatible with Google servers.

Things were still working for the most part until Google decided to stop allowing communication with external servers altogether. The users of Google Talk just saw that a few of their friends were offline, and nothing else, and users of other XMPP servers saw that a very large chunk of their contacts were now unable to be communicated with unless they created a Google account and started using Google Talk, which many did. Those who didn't lost the ability to talk to everyone that they used to talk to on Google Talk servers. Many people ditched XMPP after this.

Then Google went on to create whatever new proprietary messengers with extra features they felt like, starting with Google Hangouts, which people then adopted quickly, due to already using Google services as a messaging platform.

This is one scenario I proposed when we were last having this discussion: https://thomask.sdf.org/blog/2023/07/07/if-i-was-meta-and-wanted-to-make-fedi-implode.html