I assume an expert mode (where you can sideload anything you want) is what you are asking for. To that I raise the standard objection: how do you protect grandma from having a criminal social-engineer her into enabling expert mode and then having her sideload malware onto her device?

I mean if you're gonna stop people from owning their device and doing what they want with it just because of the possibility of crime occurring then their are quite a few more things that probably should also no longer exist because of their potential use in crime.

That's a pretty ridiculous argument if you ask me.