Hyperscalars use a blend of storage flavours covering the whole spectrum, and for most data-heavy purposes can mix hot and cold bytes on the same device to get the right IO/byte mix. At which point you can simplify down to _"are they currently buying disks to get more bytes or more IO"_ - if the HDD mix skews far enough that they're overall byte constrained, yeah they'll be looking to add byte-heavy disks to the pool. If they've got surplus bytes already, they'll keep introducing colder storage products and mix those bytes onto disks bought for IO instead.