Why doesn't Apple simply allow other messaging apps to be used as fallbacks within iMessage? Then any/all messaging apps that want to register to this fallback API could be added by the end user. This way iMessage could be the "universal" messaging portal. Heck, Apple could even rename it to "Apple Unified Messaging."
If Google wants to provide a RCS-compatible messaging app on iOS/iPadOS that can register as an iMessage fallback, great, go for it. Then Apple doesn't have to do anything except maintain the fallback registration API.
The user interface would allow some number of fallback messengers (say, up to 5), and you could move them up or down in order of preference. So if you want iMessage to try WhatsApp first (assuming WhatsApp/Meta play ball), that could be your first choice. SMS could be one of the choices, and you can decide where to list it in the ranking.
Green color coding would be retained for SMS. Fallbacks to other apps might be purple.
What's wrong with this?