I really don't get why WhatsApp is so popular in other places. I can't stand the app, it doesn't have fallback to SMS, it's owned by Meta, and as its not standards based doesn't integrate well with other messaging systems like cars etc. The users are the product. But still, you need a mobile number to signup and use it in the first placeYes and SMS will always be the fallback of last resort when it comes to text messaging because you can still get a simple message through when data transfer is an issue with low signal strength. Adding RCS just means when data transfer is at a decent level you'll get texting between platforms that's much more feature rich and closer to an iMessage experience. This is good news for everyone that uses text messaging with a legacy fallback app like iMessage or Google Messages.
I understand that third party apps like WhatsApp are a big deal in other places and people can keep using them. In the US, however, the vast majority of people just use stock phone number based texting and adding RCS will drastically improve that experience.
I can't remember the last time I've had a mobile contract that doesn't have unlimited minutes, text, and plenty of data. It's so cheap nowadays, and across the EU its standard roaming anyway. Very strange.