First of all, we don't have that problem here in europe. People just use cross-platform messengers.
Secondly, I don't understand why a company should be forced to bring its service to a platform it doesn't care about. Apple supports the default carrier messaging standards (SMS/MMS). It's not Apple's fault that they suck. In fact Apple explicitely created iMessage because SMS/MMS were absolutely terrible.
If RCS is considered a standard (is it?), then Apple should absolutely support it and apparently they plan to do so. Seems fine to me.
While I personally don't use iMessage I'd prefer it if the service was available everywhere, but I don't see why Apple should be forced to support other platforms. Just because iMessage is popular? Imagine a world where WhatsApp was either an iOS- or Android-exclusive app. Should they be forced to develop for a platform they don't care about too? What about popular iOS-exclusive apps like Things? What about Garageband or Logic? Or Super Mario games on Nintendo?