None of us know how useful all those airplanes are to the companies that operate them. Obviously at one time they served a purpose and there is a reason they are still flying. That lift can't just disappear overnight. There is still a market for small jets, if there wasn't they wouldn't be flying. There are still some highly profitable routes being flown by 50 seaters.

Taking your argument to the extreme, why don't the airlines just fly two 777's from JFK to LAX a day? They can get rid of all the smaller airplanes and cut costs. They can also just fly one 747 a week from Toronto to Detroit and be done with all regional jets on that route and many routes just like it.