why should everyone else obide by that level of strictness of control and limitations, particularly ones that are brought upon by people who can't really use their phone well? put it in a parental control mode, have that limit functionality, instead of being like 'well...people who can't use their phones, wouldn't be able to use those features - so they need to go'.
there should be a way to make a phone secure (including via optionally disabling features - and giving those controls to the user, instead of being at whim and will of manufacturer), without giving up that functionality for literally everybody.
and they kinda did make something like that, didn't they tout their "senior mode" thing recently? assistive access. there's the solution. so put their phone into a "more secure" assistive access mode, or parental control mode with limited apps, and leave everybody else who actually knows how to do things to their own devices.
bottom line, even without sideloading or whatever, app store still exists, as does internet, browsers, voice calls (the regular phone ones) and sms. all run a possibility of getting scammed any way. if you're so concerned, might as well opt for a dumb phone, or no phone at all.