> It's also a backup mechanism in case Signal fails (like it did when the exodus happened)

I view SMS as a fallback solution. It has plenty of benefits, i.e. no app required, works on every mobile phone ever made.

That's true, but the people I talk to on Whatsapp are not (usually) in the same country as me, and cross-border SMS tends to not be included in a basic package. Especially now that UK has left the EU, that's definitely not going to be cheap. Add to this that cost increases with message length in SMS - over the internet it's basically flat if you have wifi or data (up to a point at least... :D )