I would extend the argument into lifecycle costs.
In our plant, we have done an analysis on the lifetime of the plant, the age of the control system, costs of maintenance of the equipment.
In the areas that it makes sense, we are upgrading to ControlLogix as funds become available. In other areas, we are running to failure.
What we are finding is that the PLC5 hardware is becoming more and more expensive as the years progress. Even becoming Silver Series in some areas. Don't even get me started on the IO cards.
While the same Silver Series issues do somewhat come into effect with ControlLogix, there is a direct replacement available (L55 went Silver Series - replace with L6x).
In essence, do a cost analysis. Be sure to compare you existing with future development areas. One thing that I take into account is the software costs including licensing, and on-going support. I keep our spare inventory low by selecting only a few components to use. I don't have a large array of processors and software.