Re: Hmm...

They've written lots of the plumbing for OS X themselves but the important thing is that taking in proven and stable software is often a lot better than writing everything from scratch. Other OS well known software houses have tried that and come up with buggy beta-quality software claiming it to be final.

Apple is really doing the right thing by mixing home-brewed software with proven stuff made by others and this is why they can release a major new OS release, that is stable, so often.