[chao-]

"What would happen if the player app you are using to play your offline library breaks and you have no data?"

A traditional media player breaking almost never touches its "offline library".

WMP, Clementine, Foobar2000, Amarok, MPD, Audacious, so on and so forth. Hell you can use VLC as a music player if you wanted to. What happens to your songs if any of these crashes? What happens to the data if you delete the entire application from your computer? Nothing. The data is still there. The application is separate from the data it plays.

Now if the OS has issues, or the hard drive, or a few other things: Your data might be lost, but that is almost never the fault of the "player app".