I’m not sure why the App Store and third party apps couldn’t just co-exist.

There is nothing preventing your parents from continuing to use Apple’s sanctioned distribution system even if side-loading were allowed.

Why don’t we flip this question? As a savvy software engineer, I want to use my phone in whatever way I see fit and accept the risk of using unverified software. Why am I not allowed to? Oh right, bc Apple wants a cut of any profits that app developer may receive. The iPhone at this stage is a full blown computer and should expose capabilities as such.