That is a bad idea. People who never clean or reformat their cards will lose everything new, and you can't lock new videos if they are not being recorded.
Yeah I wouldn't make that the default setting.
The idea isn't really what I'd want, but it wouldn't require much code. Just a getter+setter+Interface setting for what's already there.
Other solutions would require a new block of code.
About losing new footage: the cam beeps continuesly, if it can't record. Yes, you might lose some new footage, but I'd argue people would rather keep their locked footage instead of losing it for random fresh footage.
In a perfect world, the cam would give a notification if you're lose to the limit.
How many minutes of video do you want to keep? (What options do you want for the "Loop Recording Minutes" setting?).
Maybe "30 minutes | 1 hour | 1 day | 1 week | Unlimited". (With the minimum setting being 30 minutes, that gives you an excuse to record a reasonable amount!).
The police/laws don't care about the settings of the dashcams. They'd argue to buy a different dashcam then.
The perfect solution would be to have a setting to not store anything, apart from what's locked.
However, to make this way more convenient, there should be a possibility to switch to "lock everything until I stop it or the card gets full".
Via pressing the button 2-3x or if the G-sensors trigger locking.
So you can either have some spontaneous continues footage or if you have an accident, it'll keep recording until you stop it or the card becomes full.