FDF,
Silverstrata, you keep stating that LHR will be sold or have to close, but you consistently fail to explain WHY Ferrovial (as owners of the airport) would be inclined to either sell or close the airport. Bearing in mind it's a damned good earner for them, you really do need to properly explain your thinking on this issue.
I can only presume that for such an act to happen, the govt would have to pass a law forcing it from them. They would, of course, resist vigourously, and it would be a long drag through the courts, and even then, they would still have to be paid the market value of the site as a going concern + a premium. One hell of a purchase for a closure which would not be
operationally compulsory!
Also, for such a hub to work, it is likely that other airports might need to close too, so then you have separate cases to close down multiple owners - otherwise, faced with the inevitably high PSC for using Island Airport, and it no longer having the competitive advantage that LHR has by being closer to London, more operators would just switch to LGW.
So much as though I love the concept - or at least Foster's slightly more workable version as an engineering challenge, I can't see the politics or the finances ever working.
On the other hand, a new runway at Gatwick and sizeable midfield terminal giving an overall capacity in the region of 80m would be workable financially if there was also a suitable improvement in surface access. The politics would be tricky, but even if there are some big home owners in the vicinity, it is still a far easier sell than LHR's 3rd runway.