Napster wasn't merely a tool it was a centralized service that provided an index of infringing files and where to get them. It both directly assisted in the act of infringement it profited from same.
You used the words "unintended side effect" in quotes but I am unaware of where that comes from while I am familiar with the term "substantial non-infringing use" and "contributory copyright infringement"
Youtube-dl neither assists nor profits from copyright infringement and it possesses arguably non infringing uses.
- You can download non infringing videos. Youtube-dl works on a variety of sites beyond youtube
- If you feed mpv a url it will transparently use youtube-dl to handle fetching the media for playback. Unless you specifically tell it to this does not permanently save the video and is not materially different from watching the video on firefox.
Mpv incidentally uses far less resources for decoding video even with newly added hardware decoding in linux.
- You can consume the same video you are allowed to access on a different device or in a situation wherein you lack network access. For example one could watch and then delete a youtube video one wanted to watch on the train where you lacked internet access. This is analogous to format or time shifting which have been deemed fair use.