It's not a more complicated scenario. IIRC a company loses money if the plane is not at least half full. So there's no chance a company will send two planes if the second one will only haul 1 or two passengers.

If the demand increases and they are always booked to capacity, they will send a bigger plane first.

If the demand decreases, and they are never booked to full capacity, they will send a smaller plane or will cancel the flight.

> there are thresholds which everybody (not) flying influences

There are. But, unfortunately, flying still beats everything else on convenience for anything but the shortest trips. And even on shortest trips the distinction between a train trip and a flight might not be that obvious (if we take into consideration convenience alone).