Fifty years is a long time!! In the short term you could burn your music collection onto multiple DVD's and/or pick up another good quality hard drive-they're not that expensive anymore-thus having a back-up to your Time Capsule back-up. Preferably do not store either one of these in the same physical location as your computer with TC. Of course there's also iCloud.
Longer term, the chance that any of the currently ubiquitous formats like DVD or Blu-ray will be around fifty years from now would probably be close to NIL, so you would have to keep moving your music collection to new formats as they become available. Good luck!