Jabird, if Silver Airport was built and became a success and all the existing airlines, freight operators, LHR expertise, ancilliary industries and business headquarters shifted over there, the Thames Valley would become a series of ghost towns.
Alternatively it would be reduced to a dormitory area for commuters working up in London, attracted by reasonable transport links and the consequent fall in house prices (like Essex and Kent now).
Either way it would be as difficult to redevelop the LHR site for business/industry as it is to attract business/industry to the current areas of high unemployment.
The biggest stumbling block to LHR closure would be MPs concerned about holding on to their seats. It's a much bigger issue than people realise!