Tangential to AU/X and licensing costs, I think this is why OS X was something like a "from scratch" recreation of NeXTSTEP instead of being a straight port. They replaced the AT&T licensed UNIX core with a
new open source one derived from the 386BSD forks (and DEC's OSF/1 mach fork), and replaced the display postscript WindowServer with Quartz, both for licensing cost and performance reasons.
At least, that's my impression from comparing NeXTSTEP and early OS X. A lot of the "base layer" was totally replaced, including those troublesome licensed bits.