Let's talk specifics. Here are four programs written by Knuth, in roughly chronological order:

1. The Algol-58 compiler he wrote for Burroughs (specifically, for their B205 machine). You can read about it in many places (http://ed-thelen.org/comp-hist/B5000-AlgolRWaychoff.html#7 , https://www.youtube.com/watch?v=QeiuVNDQg4k&list=PLVV0r6CmEs... , or in great detail at http://datatron.blogspot.com/2015/11/knuths-algol-58-compile...). This was written in the summer of 1960, debugged by Christmas, and put on their computers. The machine didn't sell very well in the US, but apparently it (and the compiler) was being used in Brazil over the next decade, successfully. I wouldn't call this "worthless" by any means; it did its job. (Of course one might argue that the prevailing model at the time was for software to get "done", so it wasn't really an exception.)

2. TeX. This is the most famous example. After developing it for about 10 years, he declared it done (https://www.tug.org/TUGboat/tb11-4/tb30knut.pdf) except of course for bugfixes. (He still looks at bug reports once every few years (https://cs.stanford.edu/~knuth/abcde.html#bugs), but there was exactly one bug reported during 2007–2013, and it was a really inconsequential one about the whitespace for how an "empty" macro would be printed in the error logs.) TeX is stable, well-understood (at one point of them there were hundreds of people who "knew" the entire program, which is unprecedented for a program of that size), and is very well-supported (see TUG, tex.stackexchange.com etc) — and in any case most of the questions these days are about LaTeX (a set of macros, with horrible error-handling, the opposite of TeX) or other packages, not about TeX (the program) itself. At Knuth's request, extensions are released as new programs (pdfTeX, XeTeX, LuaTeX) etc, and TeX stays the same (and even these programs have approached stability). This is definitely neither "inherently unsupported" nor "close to worthless" nor "probably won't be used in a production setting" — at any given point of time several publishers are using it in production, not to mention various others who are not even making physical books.

3. The Stanford GraphBase. This is a suite of programs, also published in book form (as literate programs). There are people still making use of these books, and these programs can be used as building blocks for other programs, e.g. for many of the ones that Knuth writes mainly for himself (see https://cs.stanford.edu/~knuth/programs.html or https://github.com/shreevatsa/knuth-literate-programs/tree/m...). I don't think continuing to work on it, versus calling them done for now, would change anything about them.

4. Any of the programs on that page, e.g. say SETSET, which was written in February 2001, and “Enumerates nonisomorphic unplayable hands in the game of SET®”. Or the first two (SHAM, written December 1992, and OBDD, written May 1996) — both written mainly to find that there are “exactly 2,432,932 knight's tours [that] are unchanged by 180-degree rotation of the chessboard”. Once the job is done, just what is to be achieved by refusing to declare them “done”? (Incidentally, note that Knuth did figure out an improvement to SETSET, but wrote it as a new program. The original program is fine though.)

In fact, if you look at the blog post by Joel Spolsky from 2002 on “Five Worlds”, in at least three of them (and maybe four), software can be quite commonly declared “done” (always, of course, except for bug fixes: “done” just means we've finally decided what the software is supposed to do; it's not the same as abandoning it even when it's not doing what we decided it's supposed to do). Throwaway code is often indeed thrown away, games get changes released as sequels or separate expansion packs, embedded software often cannot be updated anyway, and internal software can also often be "done". It's only the first one in which it is usually not.