> If such a switch exists, EVERYTHING will use it.
The switch exists on macOS already, and this is not true. Software is available from the app store. People can and do choose it. Signed software is available outside the app store. People can and do choose that. And those who want to ignore warnings or flip the switch can install unsigned software and it doesn't take any option away from those who prefer not to.
These options exist alongside one another on macOS. There's no reason to believe that iOS wouldn't remain the same, and good reason to believe that via force of habit the dominant "culture"/practice around iOS apps would remain much like it is today.
There is not literally zero software on the macOS app store, but it's pretty close. I don't think I've ever installed a non-Apple program via it, and I would do so if given the option. The iOS app store being reduced to the state of the macOS app store would make the iPhone significantly worse.
The switch exists on macOS already, and this is not true. Software is available from the app store. People can and do choose it. Signed software is available outside the app store. People can and do choose that. And those who want to ignore warnings or flip the switch can install unsigned software and it doesn't take any option away from those who prefer not to.
These options exist alongside one another on macOS. There's no reason to believe that iOS wouldn't remain the same, and good reason to believe that via force of habit the dominant "culture"/practice around iOS apps would remain much like it is today.