The Rotterdam service has always really been about holding on to the slots. Certainly, it carries a pretty large proportion of business passengers, but come next year, Rotterdam will only be 25 minutes from Schiphol by train. With the Channel Tunnel rail link and the HSL Zuid line in Holland/Belgium, you could theoretically do Rotterdam-London in under 3 hours on the train.
If KLM were committed to the route, they wouldn't still be operating Fokker 50s on it. Now that Open Skies are here, there are much more profitable uses for the slots.