This is already a problem. Apple banned WhatsApp for ages just because. So you are no worse off than before. Of course, with a competing store Apple will be less likely to ban things arbitrarily...
The issues with Apple arbitrarily banning things is a real issue, among others, and I acknowledge that and that these things need to be fixed. But don't throw the baby out with the bath water and make everyone's life who uses iOS more miserable along the way.
And no, some researcher finding out about that happening wouldn't really cause them to change their behavior. And even IF somebody outside of the HN bubble found out about it, I doubt the vast majority of people would change their own behavior and get rid of the apps or anything.
Apple is never going to build a perfect OS with no security holes and they'll never be able to 100% guarantee that an app isn't doing something like that. But the threat of having that activity discovered and Apple straight up removing everything from the App Store is enough to kind of force them into being a better actor than they would be if they controlled their own store. (see the incident where Apple found out about something Facebook was doing with their enterprise certificate and then revoked it, crippling tons of internal apps on top of their user-facing stuff)
1. OS patches would break apps making app users unhappy.
2. Patched OS could warn users of exploit attempts, damaging app reputation.