Vendor lock is real, but would that not be rendered moot of other players could offer a superior user experience? Especially when you consider that Facebook, Google and others have a far larger base of potential users than iMessage. As far as I understand, the only iMessage feature that competitors couldn't emulate due to first-party entitlements is the SMS fallback functionality, which while nice, is definitely a feature that people could live without.

Unfortunately, for whatever reason, every other messaging app I've used just has't lived up to the UX provided by iMessage. Spam is a major issue on all non-iMessage messaging platforms I've used. No messaging app I've used on any platform has performance as fast and fluid as iMessage on iOS. Competing products are often full with ads and useless features that clutter the user interface and degrade performance. And these platforms often have annoying restrictions on media quality. Facebook Messenger, for instance, has garbage quality images and video compression, while iMessage appears to apply no further compression on media files. Using any other messaging app, either on iOS or Android, has consistently been way more of a headache than its worth for me.