As I have said before there is more to Heathrow than just the airport. They will not need to redevelop the terminals into offices as there will be many office buildings standing empty in the thames valley that once housed aviation related companies supporting Heathrow.
They would also not be able to sell the land for anywhere near the amount you say to build houses as so many people in the area will have been made redundant, or had to move to the thames estuary area.
I really think you are underestimating the affect that closing LHR will have on that whole area.
There is a lot of construction going on at Heathrow at the moment. The airport is being transfomed into a world class facility and although people will, being british complain. It is in my opinion where people want to fly to/from and with the transport links currently being built, should be allowed to continue to grow. If it means the loss of one small village so be it. I doubt that many of the people that are against the third runway would actually want to see the airport close completely.