The problem is not the availability of the music but Alexa's ability to understand names. If I tell an Echo to play, say, Zuntata, is it going to work? if I tell it japanese band names despite the overall language I speak being English or French, what will happen? Most voice assistants choke when you speak foreign-y names and can only understand them if you set the language of the device to the right language.

A voice assistant is only as useful as its ability to understand speech. If it can't tell anything from half of my music library then it's not worth much to me.