The problem is that whether or not downloading a YouTube video can be fair use, the RIAA is alleging that youtube-dl is a copy protection circumvention tool. 17 USC 1201(a)(2) does not care about your fair use, it only cares about keeping copying tools off the market. To bring this back into the home recording of radio, it'd be as if Congress decided to ban the sale of tape decks that could record radio transmissions…
No person shall manufacture, import, offer to the public, provide, or otherwise traffic in any technology, product, service, device, component, or part thereof, that—
(A) is primarily designed or produced for the purpose of circumventing a technological measure that effectively controls access to a work protected under this title;
(B) has only limited commercially significant purpose or use other than to circumvent a technological measure that effectively controls access to a work protected under this title; or
(C) is marketed by that person or another acting in concert with that person with that person’s knowledge for use in circumventing a technological measure that effectively controls access to a work protected under this title.
I'm not a lawyer but how does youtube-dl fit any one of these criteria? despite the name, it's not "primarily designed to circumvent copyright", nor is copyright circumvention its only use (though "commercial significance" seems to be doing a lot of work here).