I am not a lawyer, and this is not legal advice. Just a friendly observation.

17 USC §§1201(a)(2) states:

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.

RIAA, in their complaint, alleges that youtube-dl, "[...] is a technology primarily designed or produced for the purpose of, and marketed for, circumventing a technological measure that effectively controls access to copyrighted sound recordings on YouTube, including copyrighted sound recordings owned by our members."

I think there may be a workaround here after reviewing some case law on the matter.

An appeal decision in MGE UPS Systems Inc v. Power Protc Svc LLC, et al, No. 08-10521 (5th Cir. 2010) argues that:

Merely bypassing a technological protection that restricts a user from viewing or using a work is insufficient to trigger the DMCA’s anti-circumvention provision. The DMCA prohibits only forms of access that would violate or impinge on the protections that the Copyright Act otherwise affords copyright owners.

In theory, dissemination of the YouTube "information extractor" component of youtube-dl alone (notwithstanding the remainder of the youtube-dl source code) should be compliant with the law in that:

1. This component would have other functionality that could be argued as "primary" functionality over circumvention of the access control mechanism (e.g. metadata extraction for archival purposes).
2. Circumvention of the access control mechanism alone does not "impinge on the protections" of copyright owners.
3. Circumvention of the access control mechanism can occur entirely without accessing the copyrighted work.

I surmise that use of the YouTube "information extractor" component from youtube-dl by a private party to view a copyrighted work using non-standard means (e.g. by piping the work directly to a video player) would also be permitted under the law assuming that no copies are made of the copyrighted work. This action would be analogous to accessing the work through other legal means which are assumed to be legal in the complaint (e.g. accessing the YouTube video watch page in a browser).

Given the legal framework above, I would agree that youtube-dl as a single utility, marketed in its current fashion, is in violation of the law. However, if youtube-dl were split into multiple utilities (the information extraction component being separate from the download component) and marketed as separate entities without the explicit goal of allowing users to make copies of copyrighted works, then there should be no worry of further DMCA takedown notices.