Ma Bell prevented anyone from using their entire communications network unless you bought phones from them.

Apple lets anyone who buys their devices use any messaging service they want, with no OS-level restrictions on the capability of those messaging services (that is, a third-party app can effectively take the place of iMessage with no loss of device or app functionality).

Apple happens to provide a first-party messaging service which any user is able to completely ignore in favor of one of its many competitors, and many, many do (see literally every place other than the United States).

I don’t see how Ma Bell and iMessage are in any way analogous whatsoever.