I could be wrong, but I don’t think it’s a replacement the way you want it to be. RCS requires Wi-Fi or data. SMS can go over voice channels. Google messages will fail back to SMS if data and Wi-Fi are not available.
It’s not plain text sms in either case. Apple just defaults to iMessage if the “text” is sent ios to ios. At least with signal there’s no chance of it failing back to sms.
I could be wrong, but I don’t think it’s a replacement the way you want it to be. RCS requires Wi-Fi or data. SMS can go over voice channels. Google messages will fail back to SMS if data and Wi-Fi are not available.