would tape be cheaper? it'd be a lot more annoying to use, but at least you'd have a backup, even if you have to queue a request to access.

Tape (~$6/TB) is cheaper than archival storage (~$14/TB) but I'm not sure it's worth the cost of having another "offline" system that needs robots or humans in order to access for rebuilds, maintenance, verification etc.

I'm surprised by how magnetic storage continues to decline in price as densities continue to increase. I wonder where the end actually is.

You might be surprised to see the scale of LTO tape consumption by FAANG companies.

Because at this scale the lower cost is worth it. For smaller scales the infrastructure and management overhead of tape is never repaid.