You might squirm at using refurbished or used media but those 3TB SAS ex-enterprise disks are often the same price or cheaper than tapes themselves (excluding tape drive costs!). Will magnetic storage last 30 years? Probably not but they don't instantly demagnetize either. Both tape and offline magnetic platters benefit from ideal storage conditions.
It's not just cost / media, though. Automated handling is a big advantage, too. At the scale where tape makes sense (north of 400TB in retention) I think the inconvenience of handling disks with similar aggregate capacity would be significant.
I guess slotting disks into a storage shelf is similar to loading a tape changer robot. I can't imagine the backplane slots on a disk array being rated at a significant lifetime number of insertions / removals.
If you're ok with individual storage units as small as 3TB, then we're talking about a different set of needs. At that scale, whatever you can lay hands on is probably fine. Used tape is also cheaper than new. IA is dealing with petascale, which is why I mentioned that the price difference widens with scale.
