Their is no need for new app stores. Just the option to install apps from anywhere. Like the .apk files on Android. They would not be dangerous, because they could still only do what iOS allows them. If they can hijack iOS, then there is a problem with iOS that Apple needs to fix.
Ok you install Snapchat Hack Plus you give it access to your contacts/photos that go straight to the darkweb. Let's not even look at a bad actor scenario lets look at Facebook downloading your whole camera roll without your knowledge. If they get caught oh oops it was a feature we were testing that accidentally got released. No rules have been broken but you just impacted everyone in your address book/camera roll regardless of if they sideload themselves. I would love the sue to heck out of someone for doing this in future.
How about that Facebook VPN app that recorded everything your device did and also installed a root ssl certificate to decode/record all traffic coming in and out of your device? Once again following the rules if you allow it.
Nothing in iOS could ever prevent this scenario if Apps are on alt stores. Get it now?
iOS is a safe space for those not technically savvy. I don't need to worry about a billion iOS users installing that app that's gonna affect me by proxy.
When laws catch up and you can hold individuals accountable for mistakes maybe I'll be ok with it. Right now most laws including the amazing GDPR would hold the creator of the malicious app accountable. Oh darn they are in another country nothing we can do. Nope new rules we have records from Altstore XYZ that you downloaded this compromised app and leaked this persons phone number they have to be notified and can take legal action against you.
Now if people were held accountable for their actions do you think the Altstores would exist?