In extremis, you need to archive the means of reading the data as well.
This is more true the less standard your media is: 1600bps tape using Unix tar format is much easier to read today than some 1990's commercial backup program on some proprietary, hardware compression implementation using some oddball tape-in-cartridge media.
You still have issues with, eg. melted rubber on tape rollers, leaking capacitors, etc, etc, but that's a more tractable problem than finding the right weirdo media reader 50 (or 100, or whatever) years after it was obsolete.
