Ultimately LHR will become too small, even with a forth runway. The fact it’s being discussed now is a good example of forward thinking. With growing population, especially in London and the S.E. not to mention business growth and more people travelling to more destinations, another airport will become a must.
Where it’s to be sited is still a matter of speculation, though I favour an airport in the N. Sea linking The Netherlands, Belgium and the UK. Such a massive infrastructure project would employ hundreds of thousands and help kick the UK out of the economic doldrums.
As to LHR should it close, the land it’s on and the development potential is probably worth twenty times and more the current market value cap of LHR, which would easily compensate all and provide the cash to fund a new airport.