HSM is a neat technology, and lots of ways it has been implemented over the years. But it starts with a shim to insert some other technology into the middle a typical posix filesystem. It has to tolerate the time penalty for data recovery of your favored HSM'd medium, but that's kind of the point. You can do it with a lower tier disk, tape, wax cylinder, etc. There's no reason it wouldn't be tape though, tape capacity has kept up and HPSS continues to be developed. The traditional tape library vendors still pump out robotic tape libraries.
I remember installing 20+ fully configured IBM 3494 tape libraries for AT&T in the mid-2000's. These things were 20+ frames long with dual accessors (robots) in each. The robots were able to push a dead accessor out of the way into a "garage" and continue working in the event one of them died (and this actually worked). Someone will have to invent a cheaper medium of storage than tape before tape will ever die.
I remember installing 20+ fully configured IBM 3494 tape libraries for AT&T in the mid-2000's. These things were 20+ frames long with dual accessors (robots) in each. The robots were able to push a dead accessor out of the way into a "garage" and continue working in the event one of them died (and this actually worked). Someone will have to invent a cheaper medium of storage than tape before tape will ever die.