Earlier quoted context omitted.
That's a nice history lesson but sideloading alternate appstores (namely F-Droid) on Android works great and Apple shouldn't be allowed to forbid the same on iOS. And I don't give a damn about the "grandma conned into sideloading scam apps" scenario. Grandma is getting scammed over regular phonecalls already anyway.
So to extrapolate, you're fine with your grandma being scammed via a new avenue because she's already being scammed in other ways?
Put tight controls in place. You can make it extremely difficult to bypass. You could have a recovery mode like Android and bury the setting in there. I think that makes it sufficiently hard that you don't have to worry about a grandma being tricked into rebooting her phone while holding down a specific button combination and then navigating through a bunch of arcane menus without touch controls to enable side loading. But its enough that someone that is technically inclined and who wants that control over their device can have it.