You can run that argument the other way. It's desirable for there to be only one system of phone lines installed everywhere, where anyone with a telephone number can call anyone else. It would be desirable for there to be only one e2e encrypted "double-ratchet" messaging protocol, for the same reasons.

In the case of Bell, the universal lines already existed, the problem was that Bell owned them all and wouldn't let anyone else use them. That was a "monopoly" so there was an argument for breaking it up so that everyone could use the lines.

In the case of iMessage (and others), the creators are deliberately building out many different incompatible "lines", and not letting anyone else use those lines. And now people are coming along and saying because it's not a "monopoly", we can't do anything about that. But the real goal was the same in both cases - making universal communication possible. That's what we're all frustrated about here, and the fact that there are dozens of apps trying to wall off competitors is part of the cause of that. Apple is a target because iMessage is the largest such "private line" in the United States.