This is so, so true, that I wish there were enough beer in this world to gift you with. There's a lot of cruft in XNU, and there's even more of it in the rest of the system,
but all this heap of hacks isn't just useless cruft that we'll be better off without. That heap of code also contains almost twenty years' worth of bugfixes and optimizations from more smart engineers than Apple can hope to hire and get to work together in a productive and meaningful manner. All this unpleasant cruft is what keeps the system alive and well and the users happy enough to continue using it.
More often than not, systems that get boldly rewritten from scratch end up playing catch-up for years. Frankly, I can't remember a single case when a full rewrite with an ambitious timetable wasn't a full-scale disaster. The few success stories, like (what eventually became) Firefox have taken a vastly different approach and took a lot more than users would have wanted.
A lot of idealistic (I was about to write naive) engineers think it's all a matter of throwing everything away. That's the easy part. Coming up with something better is the really hard part, and it's not achieved by just throwing the cruft away. If you innocent souls don't believe me, come on over to the Linux side, we have Gnome 3 cookies. You'll swear you're never going to touch anything that isn't xterm or macOS again.