Earlier quoted context omitted.
Ma Bell had a monopoly. This is one messaging app on one brand of phone. Nothing stops you from using another phone, or multiple or other messaging apps. And messaging apps aren’t restricted by a “default” in quite the same way a web browser is. I have friends on WhatsApp and Signal and Instagram and I message them all without any appreciable difference in effort.
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.