Yeah. I used Textra for years, and was confused why it was taking so long to get RCS. Finally decided to look it up and learning that it wasn't an open protocol yet. It's frustrating.
I have switched to Google messages, and it's been nice to text people who don't know enough about messaging to use a different app. It's only nice because Google's Messaging app is so commonly the default though.
It needs to be open and available in all apps that support SMS.