They don't even need to push it on the carriers. They can just implement their own siloed iMessage clone in the stock Android Messages app. They don't need to integrate with any carrier services to do so. Hell, simply moving Google Chat into the stock Android Messages app, and seamlessly switching between SMS/MMS and GChat (like Apple does between SMS/MMS and iMessage) would do the trick. (To be clear, I don't want them to do this, but they could.)

And even if Google pushed a new, better standard (than RCS) on the carriers, Apple could (and probably would) still refuse to implement it.