That’s pure wishful thinking. The vast majority of users wouldn’t even know what line level is, and you can’t expect end users to have audio engineering expertise. You also can’t expect anyone other than an audiophile or actual audio engineer to be able to get alll of their consumer electronics conform to such a standard
Agreed, perfectly reasonable precaution so long as it’s possible to calibrate it per device
I get really irritated when my phone limits volume with a notification like this, because the phone has no idea what hardware I have playing the sound. They’ve made some unfounded assumption about how loud 80% volume actually is, and interrupt whatever I’m doing to complain about it
Given that we’re discussing the behaviour of phones, I’m quite certain that there was never a time when they generally had line out ports. Also, I can’t imagine people are connecting their Bluetooth speakers to the wrong interface.
What you’re describing is still wishful thinking, because there’s no world where every consumer device is going to have accurately calibrated volume regardless of whether there’s a protocol which specifies it.