Someone's making the wrong comparison. You need to look at the cost of replacing the disc versus the value of the data on the disc. I suspect the disc is tiny in value, compared to that of the data it holds.
No, that is the wrong comparison. If you have data that you can't afford to lose on one device (or even one array) that is your problem - if you have a backup of the data on a drive the value of the data on the dead one is meaningless.
However, that still isn't the point they are making. It is being taken as read that the data must be protected and in that sense your point is the very opening premise of the study. They are not arguing over whether data should be protected but the most cost effective way of assuring that.
Having said that I'm still not convinced the comparison is valid. I'll admit my experience is at the lower end of the scale, only going up to a few tens of terabytes but in my experience the cost of the drives is usually around half of even the capital cost of the array. You have semi-fixed costs such as computer smarts and software on top but the extra costs per unit are not inconsiderable, i.e. physical enclosures, controllers and power supplies, which inevitably scale with the number of drives.