Connecting many disks
I see the biggest problem as connecting the thousands of drives you need to be able to compete with large tape libraries.
Even if you use SAS with multiple levels of expander, the limiting factor will be the number of SAS adapters in your controlling system(s).
With the tape archive and backup systems I have seen and used, it is possible to have multi-petabyte storage libraries with thousands of tapes controlled by a couple of systems (more than one for resilience). I'm currently working on a system with three systems connected to a library that backs up ~10TB of data per night.
I also use a high-density disk farm, with ~4000 disks controlled by 20 systems via SAS, and it is the most troublesome component in the environment, but they are always on and are configured for speed rather than capacity (although they do that as well). I will say that we get more trouble when the disks spin down and back up, so I would worry that you could have problems with not knowing about drive problems until it's too late, even using RAID. You would probably want to spin the disks up on a regular basis to detect which fail, so that you can replace them.
Of course there are trade-offs. The speed at which you can restore the data is dependent on the number of tape drives you can access concurrently, and for disk will be greater for multiple systems each with many disks, but would be more expensive