And that´s why these variants (the same goes for the Fenix Airbus, when an A321, A319, and more are released) should be made different to keep them interesting.
Having the difference only in “one flies Mach 0,84 and the biggest and most modern one flies Mach 0,86 because of the winglets” (and no other differences at all in cockpit: avionics, cockpit layout, instruments, cockpit panel layout, and even all cockpit textures EXACTLY the same in all variants no matter if Airbus A319 or A321 or Boeing 737-600 or 737-900 are chosen in the menu) will make people somehow unmotivated to buy any of the aircraft variants.
But bringing for example an older 737-600 EIS cockpit for one of the PMDG 737 variants - yess!!! 
AWESOME!
Adding some EIS into the cockpit to check the turbine status in some for the nineties ultra-techy way in the older smaller 737 variants would break the ice for many indecisive buyers 
