I remember the days when, if I had the most recent version of RS Logix5, I could connect to any PLC5 processor out there, no matter the firmware version.
ControlLogix and CompactLogix hardly even qualify as product families. They're merely brand names now, applied to a myriad of different equipment that is not always completely compatible.
It's pretty easy to convince a client that the old AB PLC2, GE Series 9070 or Modicon PLC system that they've been running for the past 25 or 30 years has to be upgraded because it's obsolete. It's a much harder sell when you tell them that the ControlLogix and CompactLogix equipment that you replaced that old stuff with just a few years back is now also obsolete.