> How would they get access to OS resources, services and share data with other apps, e.g through the clipboard?
Through the same system APIs that exist right now. Why would that change?
> Who would verify them against malware
The distributor of the app, most likely. If you downloaded a game though Steam for iOS or whatever, and it had malware, that's Valve's fault.
If you went to virus.com and downloaded a virus, that's your problem.
> or ensure they didn’t violate security constraints?
You mean ensure they don't violate one of the operating system's security protections? That's called finding an exploit, and it's the developer of the operating system's responsibility. Exploits for iOS exist today, and they'll continue to exist in the future.
> It’s not like Apple could disavow all responsibility for any data leaked from the system, a lot of users simply wouldn’t see it that way
Of course not. A "leak" due to an exploit/vulnerability in iOS that Apple failed to patch would be their fault.
A third party app leaking personal info online would be the third party developer's fault. People didn't get pissed at Apple when Facebook leaked all that data a ~week ago.
> There’s a lot wrong with the current state of apps in the App Store, but right now at least I know who’s job it is to get it fixed.
It's their job to get it fixed. It's been their job for over 13 years, and they've failed at it again and again. It's about time they're fired.