Sandboxing doesn't really mitigate much of the issue. Apps often need access to shared storage as well as contacts etc. Even if they don't need access to what really should be private, they'll ask for it. And as we've trained a generation of Windows users to do, they'll grant access. Then we have privacy breaches, or tracking issues etc etc. The OS on the mobile device may be fine, but your data won't be.
(And the platforms that don't have effective sandboxing need to implement it, regardless of whether they allow outside apps or not).