Consider the history of major software projects, and how many employees were required. BSD unix was a university project, developed by faculty and students. Linux was developed initially by one person, and then a relatively small team. NeXT was developed by a fairly small team over a relatively small amount of time. Mac OSX was basically the NeXT system ported over to the Mac platform; at the time, Apple had about a tenth of the employees of Microsoft, and was under significant financial stress. iOS and the iPhone were developed by a very small team within Apple (20+ employees if I am not mistaken). The interesting thing is that all of these systems have displayed remarkable stability and security. This likely has something to do with the fact that these OS's are all unix derived. However I find it interesting that such excellent products did not require large numbers of programmers to develop.
Contrast the above with the offerings of Microsoft over recent years. Most especially consider Vista, Win7 and Win8. During the development of these systems, Microsoft had a huge number of excellent programmers. Why did it take them so long to develop these operating system versions? Why has MS had such difficulty porting over to different processor architectures, such as ARM? Apple has had no such difficulty, porting OSX/ iOS from PowerPC to Intel to ARM. I believe that a fundamental cause of the difficulties that MS has experience with Windows lies in the early stages of operating system development. Whereas the systems based on Unix were built on a solid and proven foundation from their earliest versions, Microsoft has from a very early stage shown a tendency to build its own early versions on its own unproven architecture, with the intention of fixing the significant problems later.
Early versions of Windows 95 had very limited networking protocols, that were intended for home networking only. Wide area networking was added as an afterthought. Contrast this with unix variants, which are based on an architecture that grew up in an environment of university main-frames with hostile tinkering computer science students vying to break the system.
Anyone who worked with Windows 95 can attest to the buggy mess that it was. I supported people using it, and I remember the problems. User says, "my system crashed so I rebooted it. It still didn't work so I rebooted it again. It still didn't solve the problem." Tech guy responds, "well there's your problem. Tap the computer twice, pray to the god of your choice, and reboot it a third time, and it should be fixed."
Windows XP, Vista, 7, and 8 all originate from that same architecture, right down to the fact that they all share the engineering disaster that is "the registry". How can Windows ever be truly solid when it is built on such a bad foundation. I believe that the reason why Microsoft has had such difficulty building a solid OS stems from this weak foundation. It explains why it took MS many years and a staff 10x that of Apple to build the marvel that was Vista. As Mythbusters showed, it is possible to polish a turd. However it takes a lot of effort. And in the end, you still only finish with a polished turd.