RCS is an open standard that states to be formalized over ten years ago, and was planned to replace SMS. However, SMS is a service offered by the cell network providers, so these were addressed to get together and implement it in all networks, so everyone can reliably use it, just like SMS works everywhere. it's not a feature of phone or OS manufacturers. They simply use the SMS protocol that is already available in the network implementation.

Sadly, the big network providers have failed to reach a consensus on if and when and how to implement RCS. Some say they simply had nothing to gain from it, and I believe they might be right. So they dragged their feet so long that Apple implemented iMessage to offer more features. Google held out longer waiting for the providers, but ultimately gave up as well and implemented their own messaging solution. And while the implementation is proprietary, it's still based on the open RCS standard, unlike the fully closed iMessage protocol and implementation. This means that as long Apple supports the RCS standard, it matters little to the end user if they build their own proprietary implementation. Google and Apple will be able to talk to each other via RCS. So why is it still not great, even if everyone will be able to use RCS this way?

Well, look a bit further. Any non-Android and non-Apple phone will not be able to participate unless they submit to Google it Apple. No more indie phones with truly independent RCS. The services are also owned and controlled by Google and Apple, two companies that are notoriously resistant to regulations. Unlike cell network providers, which are under comparatively strict federal regulatory control, and even international treaties on minimal requirements for consumer protection. They are well supervised and have to adhere to high security and privacy standards. It's definitely not perfect, but the current development is much worse. With RCS, it's now two global supranational companies that will handle your private communications, and history has shown that they are much less worried about adhering to local national laws.