Tape is reliable and suitable for long term archiving, but it still needs care and feeding.
Having some kind of parity data recorded so losing a single tape does not result in data loss, routine testing and replacement of failing tapes, and a plan to migrate to denser media every x years are all considerations.
Spinning rust just feels simple because the abstractions we use are built on top of a substrate that assumes individual drive (or shelf) failure. Everybody knows that if you use hard drives you'll need people to go around and replace failing hardware for the entire lifetime of the data.
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.
Having some kind of parity data recorded so losing a single tape does not result in data loss, routine testing and replacement of failing tapes, and a plan to migrate to denser media every x years are all considerations.
Spinning rust just feels simple because the abstractions we use are built on top of a substrate that assumes individual drive (or shelf) failure. Everybody knows that if you use hard drives you'll need people to go around and replace failing hardware for the entire lifetime of the data.