Successful stores shouldn't use failed businesses as their comparable for valuation.

Those stores may have failed because of location, out-dated business models, etc. Basically for reasons unrelated to just being a "big box" store.
On the other hand, some of these big box stores may have an argument as brick and mortar has been in a decline for years so their property may actually be less desirable right now and the large supply of similar properties is pushing prices down. The 85% valuation that they are settling for seems fair but obviously needs to be reviewed on a case by case basis.