There will be more privacy, because there will be more Signal-to-Signal chats.
Also, SMS support is vastly less secure, yet Signal users loved that, and there was backlash/mass exodus when they dropped support.
and there needs to be a reason for people to switch; what’s that then?
As I already stated: privacy. People want privacy, but they also want people to talk to.
If Signal can only speak to Signal, nobody will get Signal because then you can’t talk to anybody. And there’s no point of a chat app where you can’t talk to anybody.
Add interoperability and suddenly people are more willing to try Signal because you won’t be a social outcast with nobody to talk to anymore. Suddenly you have a few contacts that are on signal and find the app convenient to use. Then the users grow from there. Meaning more Signal-to-Signal chats.
I’ve started that 3 or 4 times now.
It’s incredible that everybody was fine with SMS in Signal behaving this way, which is vastly less secure.