“When you use Signal, your data is stored in encrypted form on your devices. The only information that is stored on the Signal servers for each account is the phone number you registered with, the date and time you joined the service, and the date you last logged on.”
Have unsecured messages be opt-in and have a warning banner on non-encrypted messages. Maybe even a confirmation dialog.
That way people who want or need to be that paranoid can be, but the rest of us can have something a bit more convenient.
By disallowing SMS messaging they’ve just made it so a lot of people who were being secure when their contacts allowed, aren’t being secure at all.