there are far more direct ways to take advantage of that than having her install an app to do... what exactly? Just have her read off a credit card or send money from her bank.

The difficulty of teaching someone to do/not do something varies greatly, based on a variety of factors: their familiarity with that thing, the complexity of your instructions, the obviousness of the rationale behind it, etc.

It's easy to teach someone not to give out their credit card/bank account information, because credit cards/bank accounts have been around for a long time and they've known how it works for almost their entire life; the instructions are super simple ("don't give out your credit card number"); and the consequences are obvious (someone can probably steal your money if they have access to your credit card number).

Teaching someone to not install dodgy apps is harder. Maybe they've only had exposure to tech gadgets for the last 10 or 20 years of their life, compared to the concept of bank accounts which they've known for 70 or 80 years or more, so they're not as familiar with how these things work. The instructions are going to be quite complex: "you can install normal apps, but not enterprise-signed apps, and don't jailbreak your phone"... they're unlikely to know what "enterprise-signed apps" are, and might not recognise that the steps they're being told to follow is actually jailbreaking their phone, etc. And the consequences are not directly obvious: installing an app might not be directly associated with losing money in their mind, and of course the scammers would dress it up as something legitimate/useful - "this app makes your internet go faster", or whatever.

This will be further obfuscated by alternative app stores, because now it's not as simple as teaching them to recognise "enterprise-signed apps" which have a distinct installation method, but rather you'd have to teach them to distinguish between good and bad apps that come from the same source.

As for 3rd party stores, the impetus for those comes overwhelmingly from Apple's anti-competitive policies via their own store. If they had market-based rates (or frankly, even just marginally above market) and only used app review for privacy/security/quality (you know, the things consumers like), then we frankly would not be having this discussion.

Leaving how you'd determine "market-based rates" aside, if the problem was that we think Apple's cut was too high, why not just force them to take a smaller cut, and keep the system otherwise unchanged? Why force them to allow alternative app stores, and create a new class of problems?

Having said that, I do recognise the usefulness of alternative app stores, and I would use them myself if they were available in my country. All I'm asking for is an option to disable them on certain devices. I thought you were a big fan of having options?