OS X is an evolution of NEXTSTEP, which was started in the late 80s. They saw that OS 9 was a dead end and Apple needed something "new" and "modern", so they went with NEXT (and for a good while there was this set of compatibility APIs called carbon, PROBABLY had a lot of mac classic code). You can still see a lot of similarities between Xcode today and what they were using on NEXT in the early 90s.
new code, old code, it makes no difference. It ALL has flaws.
Heck even the images from the "Grab" program in the recent versions of OSX have the original Grab icon from NeXTSTEP