I’m not sure implementing RCS will require it to functionally be any different than SMS/MMS. Any given client registers with the capabilities they support. Supporting every feature is not required though. Green bubbles are obviously ok. Typing indicators/read receipts could be left out. Image/video quality could still be downgraded. (I doubt this will happen since there’s no technical reason to do so just for RCS).
Playing nice with iMessage isn’t required. One of the biggest headaches for group chats is that leaving an iMessage conversation isn’t possible once one person is on SMS. Other iMessage features also don’t work once one person is using SMS. That could still exist. E2EE is already out.
This may not be the win Google was hoping for.
Lack of use of iMessage in the EU might save it. Apple could also just turn it off in the EU.