Earlier quoted context omitted.
It's like taking a rabid badger as a roommate. You can in theory keep it cornered in its own room (sandbox), but there's no way to be 100% sure it'll stay there. You can be absolutely sure that it will never stop trying to get out. App Store apps can be trusted to stick to a sandbox because they are vetted to a standard before being allowed there. There's no reason for Epic-F-Droid-Store have their applications adher…
If the sandbox can be imposed on an OS level then apps on alternative stores will have to conform to the same baseline of security restrictions.
Apple themselves have "imposed a sandbox" for Safari since the first version appeared on iOS. Still we had multiple exploits, even though they owned both sides of the code.
Now imagine a situation where you're building a sandbox and the sandboxed application is using every trick possible to try to get out.