B&W wrote
"the airways are saturated and you want to bring in more aircraft with less capacity"
Yes, that is exactly the point of the exercise. You put RJ's on the routes that are being served by fuel guzzling dinosaurs which are operating half full. Now you still move the same number of pax, but in a FULL plane, which also burns a fraction of the fuel. Some days the volume of aircraft will be higher, but for the most part the idea is to run the same service but with full aircraft.
Tell me please how running a F100 or similar at 30-40% capacity makes more economic sense for an airline than running an RJ at 85-95%!