It's not the cost of the training; that would have been borne by the airlines, not Boeing, I think. The problem is that if this plane required extra training, this would have been an additional cost to the airlines, meaning they might not have bought the plane in the first place, and would have bought the competing plane from Airbus. Also, not only is it an additional cost, it means they would have had to have a different type-rating for this plane (a pilot rated for a regular 737 wouldn't be allowed to fly this version).