It's not just Android to Android per se, but RCS depends on the messaging app, and the carrier support. So not only you need to convince the other person to use Google Messages as their default SMS app, you need to check if their carriers even support it universally (instead of just within the same carrier). It's a headache. Imo Apple did it right by taking it completely first party. When one buys an iPhone, one can know iMessage works on it. When one buys an Android phone, one has to guess whether RCS support is possible.
The only fault Apple did was not making iMessage client for non iOS platforms. Apple wants the iMessage experience to be just like SMS, with no new thing to learn. "It just works." Maybe they should make their own SMS app with iMessage support for Android.