Magnetic tapes are still being used as long-term storage, as backups for example. They are inexpensive, compact, have zero moving parts, and are more durable than optical media. All you have to do is keep them in a location that is around room temperature, relatively dry, and away from magnets.

But that's not really what tar does. It simply collects the input files and writes them to a single contiguous data stream -- a file not unlike an actual tape. It's worked like that for, I shit you not, 45 years, and it is very much a single project holding up modern technology situation. I fear to imagine what would happen if it were to change.