Of course they do. There's Signal and Telegram and WhatsApp. My principal benefit of iMessage is its relative lack of spam. (And when I do get spam, it gets stomped out fast.)
Apple Music blitzkrieging Spotify is bullshit. The lock on subscription payments is bullshit. I happen to think the App Store is fine, but I'll concede that there's a real debate to be had there. But this isn't a material issue.