It think there should be a world where there is a limit on how many planes and destinations an airline can have, actually just the amount of planes would work. Like have the max amount of aircraft at 800 or 1300. It just seems an airline having 6000 planes seems un realistic.
The number is smaller than currently largest airline, and all in all that certainly wouldn't help, it would just make people buy biggest planes and actually much less realistic.
The entire game model must be more restrictive, less passengers, less a/c usage, less profit, more costs, more problems, that's realistic.
Having caps on anything is not thought through, basically too simple of an idea. Capitalism is all about getting bigger in the possible market, and currently the market and the equipment is unrealistic, fleet sizes are just a by-product and not a reason. Symptom, not a disease.