It just feels like users being restricted to not having any incoming or outgoing communication across operating systems is discriminating.

That's not remotely what's happening though? I have only ever had android devices, but message people on apple devices all the time. I don't know or care what colour my sms messages show up on their devices, but they do show up. And maybe they have a bunch of iOS-only secret chat orgies they don't tell me about, but who cares? I can still talk to them across discord, line, WhatsApp, Instagram, fb messenger, slack, Skype, signal, telegram, irc and God knows how many other different chat apps my friends and I have used at various times. The fact that iMessage is Apple exclusive doesn't make a difference to anything, they all have a different subset of apps anyway even just the android users so i have to have all those apps installed too, and my iPhone friends have the same.