There's a biiiiiiiiiig asterisk on all tape storage, about temperature and humidity. It's not like paper that you can leave in an attic for a century and still find readable.
People restoring old tapes right now have to do all sorts of exotic things with solvents to remove the mildew and baking the tapes to make the emulsion not immediately fall off the substrate, etc. I have to imagine that at today's density, any such treatment would be much worse for the data.
So those tapes are only as immortal as their HVAC. One hot humid summer in the wrong kind of warehouse may be it.
Similarly, I worked at a place where, before I joined, a system upgrade gone wrong had caused the retrieval of backup tapes stored in a metal safe, where the safe's temperature had been below the dew-point. Neither the tape cases nor the safe were sealable against moisture. This meant they had no backups of data they were required to retain for five years. And of course, the person who attempted the upgrade resigned.
People restoring old tapes right now have to do all sorts of exotic things with solvents to remove the mildew and baking the tapes to make the emulsion not immediately fall off the substrate, etc. I have to imagine that at today's density, any such treatment would be much worse for the data.
So those tapes are only as immortal as their HVAC. One hot humid summer in the wrong kind of warehouse may be it.