which motorway do passengers use to get them into London when there is no public transport?
They don't use a motorway; they use the A127 dual carriageway. However, you seem to have forgotten that many airport users will come from the significant Essex/East Anglia catchment - the only thing that seems impressive about your analysis is the magnitude of your doomsaying, I'm afraid; you completely neglect to mention the myriad advantages of a train station within a hundred paces of the terminal which at every other time of day is perfectly functional.
I'm sorry to sound rude but if Luton made efforts to remain competitive then its cries for attention would not fall on such deaf ears.