In every other global urban area, one major airport provides the main intercontinental hub
I think this is correct.
EWR has much less intercontinental connectivity than JFK.
ORY has much less intercontinental connectivity than CDG.
HND has much less intercontinental connectivity than NRT.
The major hub has a gravity denied to the secondary airport. EWR is as big mainly because it was a major domestic hub in the massive US market for Continental. Paris Orly is to Air France long haul what Gatwick is to BA. They are both secondary hub, in the case of Gatwick, not even a hub.
Why should the London market want to catch a train to Upper Heyford when we can't get the peeps up front to venture into the terrors of "Gatwick". This is plain wrong but that is a very real perception amongst too many people. economy, made up of somewhat irrational people.