Hell, Apple doesn't have to ditch iMessage; they just have to support RCS for messaging with Android users, or group messaging with mixed Android/iOS devices.
I would also (grudgingly) accept an opening of the iMessage protocol so Google could implement it in the Android Messages app. Not ideal by any measure, and I figure Apple would never do this (and I suspect Google would hypocritically not want to do this anyway), but it would at least improve things.
The thing that's sad overall is that the current state of affairs is just a result of an anti-consumer corporate pissing match. The only losers here are the users, both on iOS and Android. And meanwhile both Apple and Google get to tout the benefits of their preferred solution as if they're both the good guys, fighting for their users. When in reality they're merely fighting for their own market dominance.
[0] Tying messaging to your carrier is just a continuation of the crappy SMS "portability" experience. Sure, most RCS backend implementations are currently provided by Google, but one thing I'd like to see would be the ability to select your RCS provider. Maybe others would crop up if this were an option, and if RCS were actually popular.