It would still be too slow simulate a HDD system. You generally have two bottlenecks in tape seek performance. The first is the fetch from the robot, which can

It would still be too slow simulate a HDD system. You generally have two bottlenecks in tape seek performance. The first is the fetch from the robot, which can take seconds. The other is spindle rate on the tape drive itself which is limited by the durability of the tape. Also, in order to make this work, you'd need many tape drives, with many robots accessing the same racks of tapes. There's a physical limit to the number of drives and robots that can generally fit in a cage and have access to the same tapes so you'll have a lot of trouble pulling this off. All this and you still haven't addressed the difficulties involved in overwriting tape. Nor does it address the normal redundancy you already need to deal with corrupted and broken tapes, or the fact that you'll need to expand your setup even further if you want multiple users of the filesystem at once. Current systems already do caching and concurrent reads off multiple tapes. Tape is still not a drop in replacement for disk. Any application you write for it has to have it's limitations in mind.