Hasn't the vulnerable code been here for over a decade? Why do people think this was an easy bug to spot? There are dozens of extremely qualified people looking for these things. I think there's a reason there isn't a Nemo Phrack article about this bug: it was hard to spot, and required a flash of insight about the competing lifecycles of objects in two different domains (POSIX and Mach).
Oh too much code, bad code, we inherited it, throwing out away won't work etc are baloney excuses without meat. All it takes is the will to hire and commit the right resources with an objective of increasing code quality. I mean take this bug itself - Apple did fix it but only after GPZ was on their arse. No reason they couldn't have reviewed it themselves and fixed it.