It seems kind of the same to me if the result isn't being distributed again: "I want to view this YouTube video on the airplane when I don't have internet access." (Let's assume no international travel is involved to avoid questions of regional licensing rights.)

YouTube's paid offering in some countries includes this feature on Android and iOS, but for computer users there's no built-in way to do it. Wouldn't it fall within the spirit of the private copying rule for a subscriber to YouTube's paid service to use youtube-dl in this way?