SOE,
Yes, I agree with all your points. I don't think 'LBI' would be contemplated without the closure of LHR, and as I said in my first post, this would involve rotating London's office geography through 180deg.
There is a natural tendency of politicians to want to build shiny new follies, and of those working within the industry to be against them. A major advantage in favour would be the noise reduction - yes, all airports havea noise footprint, but LHR's is significantly higher than most other comparable hubs. However, I'm not sure if noise alone is a sufficient driver to close an airport - it is an externality which isn't currently paid for by the airport, except for a small number of homes adjacent to the perimeter, which will already have been insulated. So I'm just not sure how you get a financial ROI from that. Yes, you would win votes in several constituencies, but you would equally lose votes from those who work at the airport or use it frequently.
I don't quite think this wold be Mirabel-on-Sea, although there is never a comparison which exactly fits the situation! Mirabel failed for several reasons, but one major factor was the removal of the stopover requirement for transatlantic flights into Canada. The fundamentals of London's geography are that more people want to travel to, from, and via her 6 airports than any other city in the world. The very connectivity which LHR provides is constrained by the two runways and the limited space in the current 5 terminals.
There is no cheap answer to this problem. I still suspect that the expensive answer does not justify its cost - but if HKG is one of the closest examples of how a city can successfully build a brand new mega-airport, then I will still take interest when the man that designed it suggests an alternative for London.