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).
The person(s) who go over or under the threshold are the ones that make a difference. If thresholds aren't breached by the amount of people changing route, then there is not a very significant difference.
As you indeed state:
"but you can still very much say the climate footprint of each passenger is (total plane print/passengers)"
You can indeed say as much, but "this is oversimplified".
Just to be clear - I am not arguing that we should not pick routes responsibly, with the environment in mind. I am merely trying to ensure we are clearer about what impacts we are actually making and that we are considering all the impacts and not just doing a simple calculation that actually misses out the many complexities of transport.
This article above is a very worrying case - where the reader is encouraged to shame people for flying, without showing necessary statistics as to whether flying is indeed much worse than other transportation methods.
If a plane has 200 seats, and 400 people are willing to fly a given route, the airline sends 2 planes. Make it 401 people, they'll send three a day. That means each person who flies or doesn't brings the count closer to the next threshold, where the number of planes flying changes.
Of course, this is oversimplified, not as instant as I described it, there are price politics and what not, but you can still very much say the climate footprint of each passenger is (total plane print/passengers). The idea is not to make super accurate global calculations but to quantify individual behaviour without employing a super computer.