As long as iMessage doesn't support federation, Apple has a monopoly on it too. If not, then Ma Bell wouldn't have had a monopoly either, since you could say mailing letters was a competitor to telephones.
Any service online is under no obligation to allow for widespread interpoperation - that would be madness.
Lol, "madness". The Digital Services Act in the EU would like a word.
https://www.bloomberg.com/news/articles/2023-12-06/apple-ime...
Messages on iPhone is interoperable with any other mobile device - it supports SMS!
and isn't RCS support coming too?