No. They will send one with 400 seats in the first scenario, and possibly two with 300 seats or one with 400 and one with 100 seats in the second scenario.
It's a very common scenario
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).