Why wouldn't a modern Napster replacement a) run on your phone and b) have music discovery? Doing both P2P is only a legal challenge (as in, it is illegal), not a technical one.
Illegal things are harder to do on phones because the app stores act as enforcers in a way that desktop OS vendors don’t. Most people don’t sideload or jailbreak.
In what world would Apple allow a piracy app on their AppStore? This may change with the EU demanding other AppStores - but as of right now it would be unworkable.