OK so let's imagine sandboxing is on by default, then "Transmission" pushes an update that asks for read/write access to the whole home directory. You don't know if Transmission has some legitimate need for that or not, so you just shrug and click Allow. Boom—infected.

So I think sandboxing is basically useless against these kinds of attacks. Either they allow apps to elevate their entitlements in an update, or they don't and developers will always out-out from the start (or pick the widest set of entitlements available).

If sandboxing is forced with no opt-out, then users will have to jailbreak their computers so they can install Parallels...