What would certainly help would be to start offering hard disks with a physical WORM setting and better hot-swap capability to compete properly in the D2D backup space. I'd throw so much money at Seagate if they could build a hard-disk based replacement for my aging tape robot-library system. Building out a box for the disks to be completely tray-less would be trivial (I have a couple such bays on my desktop), especially if they added some kind of twist-lock notch to the physical casing to aid a robotic arm to grab it (The corners of the drive are already hollow most of the time anyway).
Maybe add in some kind of RAID-like redundancy into a backup set and you'll get something even better than tape. I can't count the number of times I've had a backup set ruined by a tape having failed for one reason or another (Broken gears and dried-out / structurally defective tape being the two biggest issues).
I figure a proper system would write the backup data across a 12 or 14 disk RAID-6 (or preferably something that provides the same level of redundancy but in a more backup-friendly format) backup set loaded into a tray-less box. Each disk would have a pair of notches on the end so that a robot arm can easily grab and pull the disk. The disks would be equipped with a physical switch or a set of fuses to electrically disconnect the write heads. Perhaps add in some logic so that backup device knows how much data it will back up and can use smaller, or larger, hard disks to avoid waste.