if there was a perfectly good replacement part sitting on the shelf, then the techies would grab it first- lot's less work. The problem is the part is NOT on the shelf when it is needed.
Doesn't always work that way either. Entire squadron was unable to put up an airframe to carry out its primary role for want of a particular black box. Stores said none available, on order. AEO on making a personal check of stores found six sitting on the shelf. Can't have was the answer, minimum stock.