A lot of people I know can't afford a data plan for their phones, so when RCS becomes enabled it just bounces all their incoming/outgoing messages and it is sometimes days or weeks before they realize.
And even for those who can afford a data plan for their phones, they might have gone over their data allowance for the current month, so the data won't work until it refreshes at the start of the next monthly cycle. And obviously, having one more thing using data makes it even easier to go over the limit.
For me, SMS is solely for receiving 2FA codes and sending message to my provider to check how much data allowance I have left for the month. I use Whatsapp for messages to people.
Companies have already begun using RCS as an opportunity to flood my phone with ads that take up way too much space in the notification shade. Also not from the US, so I can just use a 3rd party app