The Pilot Operating Handbook (POH) contains numerous tables that pilots use to calculate what will be safe operating conditions for an aircraft. These include how much runway will be required at specific temperatures/elevations to clear obstacles. Those numbers were based on a 100 octane fuel so if the fuel is changed, those values must be determined once again and I believe that is largely done with flight testing. To do that on the entire fleet would be prohibitive. Without these small planes the misery you experience flying commercial would increased greatly due to the lack of pilots. It's pretty bad as it is.