Until they got rid of them KLM could only justify F50's (nothing larger) on the RTM LHR route, I can't see how F50's may be justified on lesser yield RTM routes
Quite. VLM/Cityjet dropped MAN-RTM with a F50, and MAN is quite a big airport. The service on board was very good too.
One could argue that this service failed because of the high frequency of MAN-AMS services in competition. But most airports seem to have an Amsterdam service with easyjet and/or KLM. And if it's KLM, I expect business passengers will value their SkyTeam air miles (or whatever they're called). It's not that far by train from Schipol to Rotterdam, and I have no doubt that very important business passengers may have their firm's driver pick them up or be able to claim a taxi.
A fifty seater needs some filling of economy seats and, with respect, Rotterdam is not a tourist Mecca; a nineteen seater needs businessmen and women.