Suppose Heathrow didn't exist, try writing a proposal for it as we stand today. There is no way it would ever happen.
An airport will be built somewhere in the Thames Estuary and it will become a major European hub. This will happen eventually as the political will gradually builds. It will be privately funded and the major airlines will have no choice but to go there - whatever they say in public.
It will take 20+ years, so I may see it in my lifetime (64).