A lot of people in the comments are wondering how this could possibly compete with WhatsApp, but I don't think competing with WhatsApp is the goal at all here. While WhatsApp is super prevalent across Europe, in the US, most folks are still just using SMS with whatever their phone's default messaging app is. For iPhone users, that's iMessage, and for Android users that's Android Mwessages. There's just far too much inertia on SMS for that to change in the US - everyone's already got a cell phone number that can be used to send SMS messages to, but WhatsApp accounts are few and far between.
And I gotta say, when the person you're chatting with is also on Android Messages, the silent upgrade to RCS has been great. It finally feels like texting in the 21st century. So if Google can start rolling this out as the default experience for US-based Android users, it's gonna make a lot of people happy without them ever realizing it.