So what about those that don't use Whatsapp, or do everyone you know use Whatsapp? I don't use Whatsapp and I know a few that don't either, so use SMS. Those that do, if they want to contact me by message, then they can use SMS.
As for RCS, it is supposed to be the replacement for SMS, so people can send stupid hires emojis to other people and high quality photos and videos, like what can be done in WhatsApp. The difference is that RCS will be on every phone, so no third party app required. Been around for a while now, with Google pushing it.
I use SMS, the app I use Textra, don't support RCS and i would turn it off anyway.
Does not exist. Even grandmas use WhatsApp