It also wouldn't matter much to me if developers could upgrade. What I would want is for users of my app to be able to upgrade when prompted.
Fortunately the cutoff I've chosen for my current project isn't that bad - 15.4. It means some iPhone users won't be able to use it. Eventually I'll feel OK about requiring 16.4. But I second the request for being able to install a new version without upgrading the whole OS.
Edit: Sorry, missed the part about the "mobile" safari. You're right, I don't think it's available for mobile.