Whilst Edinburgh has been remarkably successful over recent years, it can only overtake LTN temporarily - assuming the LTN development proceeds.
Air traffic in the London area is increasing far more robustly than elsewhere in Britain, and Luton is well placed to attract traffic growth if it has the capacity. EDI will gain if Scotland becomes independent, but that will not be happening any time soon, despite Mr Salmond's best efforts. Whereas there is shortage of capacity in the London area, the same can't be said for the Central Belt of Scotland.
QED