didn't they do that on one of the mac os versions too? and it turned out to be one of the best choices jobs claims he made?
Of course those are just the starting points, from that a lot has been reworked/improved and a lot has been added.
(I kid, I kid.)
In Apple's case the re-write idea turned out horribly and the only reason they made it thru is because a former founder had created a startup years earlier and had a working OS that Apple could buy and use.
Apple is hardly a poster child for re-writing software.
It looked cool, in the late 90s.