I'd disagree with both Apple and Moxie here and say, yes, for messages or anything else that induces a network effect, interop should be mandatory.

The thing is that I can't choose freely which messenger I want to use like I could with other services: I'm restricted by which messengers the people I want/have to talk to actually use.

So yeah, build your walled gardens if you have to, but then don't force me into them.

Of course forcing people into their walled gardens is something Apple's leadership is very much interested in, as evidenced by the "don't make it too easy for an iPhone family to buy their kid an Android phone" email.