If Apple should be expected to open up iMessage, then why shouldn't Facebook be expected to allow interoperability with Messenger, and Telegram and Signal forced to open those up, too?
Or, if you wish to take it a slightly different way: Why shouldn't the New York Times be expected to give everyone a copy of their paper for free? Information and good journalism are a public good, after all.
Why shouldn't pharmaceutical companies be expected to make all life-saving medications available for free?
Why shouldn't ISPs be expected to make Internet access available to all for free?
Also, Telegram and Signal are already open and can be bridged from anything to anything so yeah, we should force Messenger and Imessage to accept, at a minimum, bridging as well.
Good to know about Telegram and Signal—I don't use or know much about them. IMNSHO, my argument still holds even removing them from the equation.