The reason why Google wants Apple to implement RCS is because RCS includes a lot of what makes iMessage so popular and would bring the experience of texting someone with an Android phone up to the same level as texting between iPhones.iMessage is going no where anytime soon. What Google is effectively asking Apple to do is to do away with SMS functionality in the Messages app and replace it with RCS. I won't pretend to know the technical details in what that would take to accomplish. RCS messages would still be Green... You can take that to the bank!
To give an example right now two people texting on iPhone's can send full resolution photos and videos, bring in a third person on an iPhone and all is good you still get full resolution photos and videos; now bring in a forth person on Android with the dreaded green bubble and now you are no longer sharing full resolution photos and videos. Now if Apple implemented the RCS protocol the group message would switch from iMessage to RCS as opposed to SMS but the experience would remain the same where as with SMS the experience is significantly degraded as a result of bringing the Android users into the group chat.
So in the case where Apple implemented RCS Apple effectively removes the iMessage advantage so unless forced to they won't be implementing RCS. Besides Google is being a bit disingenuous because they are asking Apple to implement their version of RCS which includes some specialized Google "enhancements" and RCS itself still isn't a standard so Google is actually asking Apple to implement RCS with the caveat that specification will change. Then you have the fact that Google despite their campaign calling out Apple for not implementing RCS still hasn't fully implemented it into Android.