I have seen it observed that while the tr/sort/uniq approach has practical value, that if the question is how would you implement the features that tr/sort/uniq provide simply invoking them is not much of an answer. I don't think Knuth was unaware that there were existing tools that could solve the problem, the whole point was to demonstrate his literate programming approach at some semi-reasonable scale on a problem everyone could recognize. It's kinda like how pretty much every interview-sized question has some sort of off-the-shelf solution somewhere, because anything that fits into an interview has been solved thousands of times over... but that's not the point of interviews.

In modern terms, it would be something like complaining that $LANGUAGE's web server implementation is a grotesque bloated mess (and therefore $LANGUAGE isn't very good) because in shell all I have to do is run "nginx". There is a true and useful sense in which that is true, but there is also a true and useful sense in which that is utterly missing the point.