Downloading from youtube is piracy? How? If it was like a Youtube Red show, sure, but the normal videos everyone can see for free?
For me piracy begins with aquiring things or features which usually cost money to get whilst also taking into account if its obvious a thing should cost money in such an environment (thats also how our piracy laws are worded here).
So our piracy laws also classify things as piracy if it was obvious the deal was too good to be true like Windows for 2$ on eBay or chinese ROM cards for 5$ with hundreds of games.
Videos on youtube, including music, are a normal occurrence. A full blockbuster movie is usually not.
If it would be hard to do and having to bypass DRM yes, but its actually similar to what the player already does.
A court already ruled here that downloading youtube videos does not break the piracy laws by providing own means of downloading and saving the unprotected data.
Of course that does not include allowing the download feature of the client itself.