Software isn't even installed, new versions are downloaded daily or more so the concept of sandboxing as been throughly tested and proven effective for those 27 years.
And that's before we get into resource usage.
Apple should build a better sandbox, the idea that "private APIs" exist and the only thing stopping them from being used is a basic string search on the app store review is pretty horrifying.
Nope, sandboxing is largely orthogonal to performance.
I haven't done enough iOS development to know for sure, but I'm assuming Apple could prevent private API usage by apps through technical means, rather than just app review.
And also because, once you’re jailbroken, you can setup software to automatically resign the app on-device every few days, so you never need a computer again.
This, of course, is a catch-22. You need to run an app to allow unsigned apps, but that app can't run if it isn't signed.
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† The community makes a distinction between "tethered", "semi-tethered", and "semi-untethered" jailbreaks. The jailbreak I described above is "semi-untethered". You really couldn't come up with terminology more prone to getting mixed up...
There's also:
- App store guidelines on what is and is not permissible in different cases
- Restrictions against using private APIs
- Restrictions against jailbreaking the device
There are a variety of VPN apps available on iOS. Why was Onavo blocked? Because it violated the guidelines on the use of the information, which is the kind of thing that it difficult to automate.
Restrictions against using private APIs are semi-automated, and would be difficult to completely automate.
The fact that you can't get an iOS app that jailbreaks the device in order to do whatever it wants is in part due to human review - if one existed in the store, it would get pulled, and the developer cert would get revoked. Jailbreaks exist, and human review in the app store is one way they are mitigated.
I for one remember the bad old days when playing a CD could (and did!) install a rootkit.