They got to be at least 40 years old before the worst units to be considered written off to "Fish housing" and the better units can be used for few more years of storage, then run their courses to follow sue in their demise at the raise of their future successors. Unless something changes with the time, this is a never-ending 40+ year cycle with the rolling stock.
Yeah, don't go looking at the R188s and think that should be a thing with the existing fleet. With the 's financial track record, that's unrealistically null....