Interesting theory, let's just wait and see if KLM ever flies Airbus narrowbodies
. Maybe I am more chauvinist than you, but the ATR, even the -500s, doesn't offer any more comfort and technological advanceness than the F-50s. They want to get rid of the prop aircraft within a few years as their business passengers complain it's "slow", "old fashioned" "flying doctors" aicraft.
Don't forget it was Exel, who has more financial independency from KLM than Cityhopper, which ditched the ERJ as their financial situation is bad and their ERJ-suitable routes MST-MUC and -Milan didn't work, while the ATR is fine for AMS-MST, AMS-EIN and MST-London and such. For Cityhopper, a smaller regional jet, especially the ERJ, would be great for longer routes now served by the "slow" F-50, like Bristol, Cardiff and maybe new routes to northern Italy, Spain, southern France, for which the F-70, let alone a 737/A320 family member, sometimes is a little too big.