In theory, this bag would be used only as a carry-on, correct?

With that assumption, the bag should never leave your side, so risk of someone stealing something valuable from the sleeve, is minimal. And if it is theft of valuables you are worried about, just leave the item in the main compartment.

I carry an attache or backpack, as my personal item carry-on, so if there is something I urgently need it is always accessible. Once I pack my RIMOWA Topas, I never open it again until i'm in my hotel room. At that point, beyond locking the case when i'm not in the room, there is not much more else I can do to mitigate risk of theft during travel.