I had some of the same thoughts. But I learned C and Unix a long time ago, back when you could more easily understand the source code. I think we used a very early version of Minix, just a few thousand lines. It was a simpler time ;-).

Coming to it now, with all the layers that have built up -- the side-track for the script that uses tr, and the idiom with ${1+"$@"}, and all the cruft build up within sh(1) -- it must be pretty hard to separate what is incidental from what is fundamental.