>Apple has enjoyed the fact less companies target OSX and, assuming the same competent to incompetent developer ratio for both platforms (something risky in itself, because the barriers of entry for Windows are much lower), it would be no surprise if there were less buggy programs for OSX.

You can't say there's less buggy programs because it's hard to break through. You can say there's less buggy programs because there's less programs in general, which is a much more rational assumption considering you assumed that there's the same ratio for competent to incompetent developers thus making about the same ratio of buggy to polished programs.

However I don't see how Apple's Kernel really is "locked down", I interpret that as a forced cull of functionality for safety or in terms of things like DRM. The only thing I'd consider is the lockdown of the hardware because of forced lack of choice.