That's exactly what Apple, Google, and Facebook did to XMPP. They all started with a federated open protocol messaging system. At first iChat users could talk to GChat, and Messenger users, as well as users of thousands of other servers. After they built their network they closed off federation under the guise of "feature development".

To this day, iMessage still uses an XMPP based backend. But green text is for Apple users only!

Might seem naive, but I actually have a hard time imagining this. There's just not a lot to make one instance more desirable than another, which seems like a bad thing, but I don't think it is. I decided against signing up on lemmy.ml because it was laggy, so I went with a smaller instance- all the same content, but without the lag. If a lot of content gets created on one instance, there's no pressure to pile in, because you can view, comment and interact from a different, smaller instance.