Perhaps? But unless tape, and the infrastructure to support it, is dramatically cheaper than disk, they might still be better served by more disk - having two or more copies of data on disk means that both of them can service load, whereas a tape backup is only passively useful as a backup.

A lot of people, me included, consider anything online not to be backup. Being disconnected and completely at-rest is a very desirable property.

    unless tape, and the infrastructure to support it, is dramatically cheaper than disk, 

This turns out to be the case, with the cost difference growing as the archive size scales. Once you hit petascale, it's not even close. However, most large-scale tape deployments also have disk involved, so it's usually not one or the other.

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.

That's exactly what it is.

You also don't want your true backups online at all - that's the whole point.