@It's a question of speed
"""100Gbit/s transfer speeds without just speeding up platters would be a good place to start."""
I really wonder what you'd do with that sort of storage bandwidth, seeing as how (dual channel) DDR2 800MHz does about 6.4gbit/s under ideal conditions. And a 16x PCI Express slot tops out at a theoretical 32gbit/s.
Oh yeah, and the fastest sata ports currently planned can only do 6gbit/s.
You should also know that the only way to speed up a large sequential transfer (Like your day long backup example) is to increase density, which seems to be the very thing that you'd rather they don't work on at all. The problem, which you noticed, is that speed increases relative to linear density (bits under the head per rev) whereas storage capacity increases relative to areal density. That means that capacity will increase exponentially faster than speed for rotating magnetic storage media.
The good thing about increased density is that smaller drives get cheaper, and you can put them into an array, where you can actually make them significantly faster, given that you know what you're doing.
But there's no way you're going to get 100gbit/s on anything at all short of some supercomputer interconnect buses. I believe that the fastest off the shelf connections that you can guy (for a whole lot of money) are still limited to about 40gbit, although I don't keep track of those and they may have increased recently.