UK smartphone penetration is at 97% so almost everyone can already install and use a cross-platform messaging app.
Yes the networks need to support RCS for it to work. They’ll need an incentive to support it since implementing RCS will incur costs. Incentives might be the ability to sell messaging services to businesses, etc. (although businesses might decline since they can already reach most of the population via WhatsApp. There’d probably have to be a sizeable portion of the population who are only contactable via RCS to make that worthwhile).
So still 3% who don't have a smartphone, that is still a lot of people, also SMS will work on a 2G signal and also on a very weak signal, RCS need data and a pretty decent signal to work.
I know that RCS will default to SMS if it needs to, which is why we need to keep SMS going.
People keep saying about WhatsApp, but a lot of people, including me don't use it and have no interest in using it. I used to get people asking me to install it, even requests, but that have stopped now as people have realised i will never install WhatsApp, singal or anything like it.
A lot of older people don't use Whatsapp and again, these people who have a basic phone can't.
SMS does the job for me, it allows me to send messages to people I want to send messages to and to receive messages from them, I don't want people sedning me photos, videos and using silly emojis. I have seen some messages on different phones, and it is like going back to Ancient Egypt. i used the basic ones, sad face, happy face, laughing, thumbs up and thumbs down.