The exfiltration of which is as easy as exfiltration of database on device. You're not running an IDS scanning 100% of your device LTE traffic in case that happens.

>isn't that a roundabout way of replacing all of signal's protocol and its forward secrecy with a static key that has no forward secrecy?

It's opt in. And again exfiltrating the backup key is as easy as exfiltrating your messages from your device.

>You can’t know whether someone you’re talking to -- who may not understand the implications -- has enabled it

You can't know if you're talking to an informant or if your contact is running Android that's receiving security updates or if it's a zero-day on wheels, either. Tech doesn't solve human problems.