The term "bathtub curve" came from the fact that typically hard disks had a lot of early deaths in the first year right out of the factory, hence a graph of hard drive death age would have a high side on the left (brand new drives dying) and a high side on the right (old drives dying), and during the average lifetime the drives were reliable, so the curve was shaped like a bathtub (high sides, flat bottom).

This lead to a lot of people doing things like intensive burn-in cycles for a week to test new drives before putting them in their machines.

When there are no longer early deaths (the "left" side of the bathtub), it's just "old things wear out" which doesn't have a special word for it.