1) it's not just you, it's your partner too. So that's two seats x # flights/yr.
2) with the extra seat miles you are occupying, airlines can sustain their existing routes/itineraries or expand them if they're at capacity.
At a larger scope it can be viewed as: will the airline fly more flights with its existing fleet? Will the airline buy more aircraft to fly more flights? If yes to either, then your argument breaks down.