And the NT kernel has it's roots in early 16-bit architecture, which is probably contemporary with the Mach kernel you speak of.
But you could trace code lineage (of any OS) back to the stone age. OS X was a highly specialized build that used Darwin elements at it's core, optimized for modern hardware. As has been pointed out, most backwards compatibility was taken out for the greater good of the OS. It was almost an entirely new product when released.
The same can't be said for Windows, which hasn't seen a completely rewritten core since the late '80s. I haven't heard of anyone who likes Vista, in fact, 100% (that's right, 100%) of the people I know personally who have tried it switched back to XP within a week or two. I don't personally know a single person currently using Vista by choice on their home computer.
I don't actually know anything about Vista, but it seems to me judging by consumer response that the inevitable, inherent problems of using an ancient architecture without retooling are finally becoming apparent to the sheeple. I saw the beginnings of this with XP, but at the time the monopoly the Micropenis held over the home PC OS market pretty much left people with no choice. The final release of Windows 95 was the last MS OS that was any good during it's day, mainly because (at the time) the framework wasn't obsolete. And goddamn I held on to that OS as long as I could before developers just stopped supporting it altogether and I had to upgrade. I completely skipped 98, 2000, and ME and used 95 right up until XP came out. After a year with XP that included several viral infestations, two complete HD wipes w/re-installs, and a keystroke logger that stole my credit card number and cost me $1,000 in credit card fraud, I joyfully parted ways with Micro-penis-soft while happily giving them the finger until they were a mere spec on the horizon.
I won't even think about giving them another chance until they stop trying to rip people off with the same kernel packaged with a new GUI and a pricetag intended to drain your retirement account.
BTW, I eventually got that money back from the fraud incident but it was a month of hell and left more than a bitter taste in my mouth. I'm not an Apple-hugger per se, but in today's computing age their OS is far and away the superior choice. During the 90's I definitely wouldn't have said that.