Pardon me but this is another commonly used, but erroneous, argument of why rail is greener than air. City centre to city centre without the trip to the airport. But how many people are actually going centre to centre? I'd guess that in most cases either at the origin or at the destination but not so often both. For a passenger travelling from near the airport and destined near the airport on rail would be 130mins by rail vs 55 by air. (Or if we split the difference 110mins vs 75) Yes, for some journeys your comment is valid, but not for all.
With respect - those near the airport have to travel less, but those the other side of the city have to travel further. Most airports are outside the city boundaries, so everyone has to travel further than they would to reach the city centre rail station.