In 1993, Knuth requested of Stanford University (who granted his request) that he be allowed to retire early (and take up the title of “Professor Emeritus of The Art of Computer Programming”), so that he could complete TAOCP, which he regards as his life's work: https://cs.stanford.edu/~knuth/retd.html
Needless to say, the measly royalties from a technical book (especially one that is never going to be assigned as a textbook to large classes of undergraduate students) would be tiny in comparison to the Stanford professor salary that he gave up.
Also, if he were doing it as a source of income, he'd probably actually publish the long-awaited volumes so that they can sell and make money, rather than spend decades of full-time labour polishing each one (Vol 3 first came out in 1973; Vol 4A in 2011; Vol 4B is only about 1/3rd done), continually missing estimates, until they were at his desired level of quality.
Insisting on reducing every motivation to whether or not it makes money is such a narrow-minded display of being unable to imagine other ways of viewing the world that it's almost insulting.
Needless to say, the measly royalties from a technical book (especially one that is never going to be assigned as a textbook to large classes of undergraduate students) would be tiny in comparison to the Stanford professor salary that he gave up.
Also, if he were doing it as a source of income, he'd probably actually publish the long-awaited volumes so that they can sell and make money, rather than spend decades of full-time labour polishing each one (Vol 3 first came out in 1973; Vol 4A in 2011; Vol 4B is only about 1/3rd done), continually missing estimates, until they were at his desired level of quality.