Earlier quoted context omitted.
Most people are naive about how applications will coerce them into doing things against their best interest when given the ability, such as a request while installing, requesting access to parts of the system that reveal PII. Most people do not have the knowledge to evaluate how these things play out. Apple via the app store at least does some work to mitigate this and when they do fail, are able to fix it.
You are making the case for the app store to exist, not for it to have no competitors. Let's suppose that Apple's store is infallible. Well then the availability of other stores won't matter, will it? Apple will approve everything good and reject everything bad and you'll know that anything not in their store is bad and have no reason to ever look at another store even if they exist. But suppose they're not infallibl…
But first, we know the App store isn't infallible, but it has an incentive to have more false negatives than positives. It is able to correct past mistakes and does do so.
But as far as competitive pressure, that argument is mute as long as there is no way for the "normal" owner/user of a device to evaluate the market. By the time the bad actor is exposed, it is often too late. The other side is that one only has to look at the Play store to see that there are so many copies of original apps that it is obfuscating them. One cannot find the legitimate app. So until most people are able to be informed and evaluate the apps, it isn't in their favor to want alternative stores.
Also, one has a choice, buy or don't buy the device. Apple does not have a majority of sales in phones, not even close. Also, you can side load any softare you want. It's a service that is paid for , but for free it's 7 days per install.