My takeaway is it's much easier to embark on huge side projects as a tenured professor.

you completely missed the point

Agreed.

I'm also incredibly grateful --- I was gifted a NeXT Cube by my brother-in-law when I was in college studying graphic design, and I was incredibly put out by the paucity of nice typography features in Quark XPress and Aldus Pagemaker when I came across TeXview.app and remembered checking out the book _TeX and Metafont_ from the local college library when I was a senior in high school --- built that into a career and a series of presentations at TUG conferences where I got to meet him.

The TAoCP itself is a side project from his original goal of writing a book on compilers.

Gotta shave some Yaks

Did he complete the compilers book, or even start it? I know he has worked on TAoCP for a huge amount of time, and on TEX too.

From what I understand, TAoCP is building up to that compilers book! When initially tasked with writing about compilers, he realized all the background necessary for building them and planned out these books as an overview of computational techniques that would be helpful to use.

See the "future plans" section of his website: https://www-cs-faculty.stanford.edu/~knuth/taocp.html has volumes 6 and 7 as being on compilers.

danke, freund