Adding to that, is a common fallacy of "the plan was going to fly anyway, it wasnt full, so my CO2 contribution is small".
The reality is that the more passengers on a route, the more the airline will be enclined to increase frequency. In other word, as an individual, the CO2 footprint appears to be only the one from the added weight. But collectively, this send a market signal to increase frequency, with larger CO2 impact. At the end of the day, it only take "one extra" passenger on a given route to cross a threshold, and justify for the airline to flick the switch and increase frequency.
It s a bit like voting. While noone's indivual vote will make a difference, collectively, the impact determine the future of a population.
It's a fair point to be sure, and I aluded to it elsewhere myself.
The thing is what are the numbers involved here?
Airlines typically look at demand as avg pax/day over a certain period - perhaps monthly or seasonaly.
One extra may change say a 130avg ton 131 as example, but it still wont massively change the demand profile.
You'd need significant uplift of "unnecessary demand" (for want if a better term) to push the demand profile to warrant looking at either an upguage in capacity or extra services.
And, that may mostly inly be vaid for domestuc more than most international ops where other factors come into play such as slots, aircraft availability, bilateral agreements of capacity caps etc.
Even on domestuc, fleet constrants, airport capacity, gates, fees, etc all come into play.
Further, folks booking flights just to run are still restricted by inventory and yield management on routes. I doubt many runners buy full flex fares, so the effect may be on reducing existing inventory for others (and potentially higher prices in some instances).
If one assumes runners tend to discounted fares (even in premium cabins) that paints a different picture I reckon and lower yield traffic demand may not result in additional capacity, but probably tighter yield controls to try to increase revenue.
Tbh I don't think pure mileage runners are more than a minor blip of total trips and probably do not influence demand profiles much.