Jury is out for a while yet-- too long in my view. But it won't be just one thing in that scenario. Could be a combination of
bigger planes on the hub to hub routes
increased market share for point to point from LGW,MAN,BHX using 787s etc as market size expands. No-one goes through a hub just for fun, maybe traffic will spread out a bit naturally if there's more choice.
More hub competition-- maybe more DXBs are coming anyway.
get the break even point between air and high speed train out to 500 miles for business and S France, Barcelona etc for leisure
more capacity (if not more runways) around the fringe of London and SE to cater for point to point growth
Not saying that's a good solution but might be the sort of thing which happened if it's a pig in a poke.
Is there a good book on the New York airline/airport system? Strikes me we are in a not dissimilar position to New York. Great city, legacy airports, very difficult situation