Or I don't think computing freedom has to be completely abandoned to protect someone's grandma.
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.
Scams exist because the scamming industry is a large fraction of gdp in some countries and the byzantine financial system doesn't allow for reversing charges.
My grandma has an android phone and I'm fine with her having a phone that could permit her to sideload an app. Having a phone number at all is a far more serious threat, and I presume you are fine with your grandmother having one even though a scammer could talk her into giving up her bank details or buying dozens of gift cards and reading out the codes.
The solution to the grandmother scenarios is to have a trusted relative that works closely with them, who they trust to copilot or handle completely all business dealings. If that's not possible, then they're at risk whether they have an iphone or android.
