Why couldn't this be solved by Apple simply adding a feature that allows you to lock the installation sources behind a PIN? We've had these type of restrictions for other subsystems, so I don't see why it wouldn't be possible here? (Modulo security vulnerabilities, but those exist for the current iOS ecosystem as well).

Example. I could set my parents' device up so they can only install from curated stores (Let's say App Store & Epic Games store, if they were inclined to play games). Now I don't have to worry about them being scammed into sideloading shady apps, they don't have the PIN; and I get all my freedom to install whatever I want.

I'd be honestly happy to hear an argument for why this wouldn't work, cause I haven't been able to work it out.

edit: In a world where apple is mandated to provide sideloading access, it's even in their interest to provide such a feature. It allows them to hold up their "we're security focused" claim, while at the same time potentially increasing revenue (in my example, my parents are still vendor-locked, just by me, their "digital guardian").