Spotify can play online. However, if you want to download songs to your device, you can play them later offline too. But having lists of songs in your phone will occupy a big oart of your phone memory. This is contradicting. If you want it offline, it can be at your phone memorys expense. If you want it no installation you will need a full internet connection
I think that is the reason why they will be using Progressive We Apps (PWA) for this. Imagine that PWA will be using 1% of your storage compared to native applications. This is amazing, right? How will they do that? I don't know for now how they will compress songs but one thing I am sure. They can do that for sure!
I think in that case we'll have no choice but to connect to the internet if we don't want to save it and use your phone memory or vice versa, but i do hope yappadappadoo will have apps that somehow can listen to any songs we wanted without saving it to our phones coz im a music lover...