That’s not how it’s supposed to work. It’s a federated system with each carrier supposed to run things for their customers.
Why does it work today? After years of begging Google gave up and put all their users without carrier support (basically everyone) onto a Google RCS instance.
Despite how the article tries to portray things in many ways it’s iMessage but worse. With Google running everything it might as well be proprietary. And it lacks full E2E encryption.
Things probably would’ve been better off if Google had just stuck to their own private protocol and used that instead, perhaps opening it.
It's not just on the telcos, Android fragmentation as a whole makes implementing new network features an uphill battle. VoLTE is an absolute mess (on Android) if your build doesn't have settings for your carrier. Google doesn't sell the Pixel in my country, so the only way to enable VoLTE here is to root your device. This is only going to get worse with carriers sunsetting 3G networks - anyone roaming on a Pixel in such a country won't be able to make calls, even to emergency numbers.