You missed the point if these multi R/w airports can spring up in the desert without any discernible local traffic then so too could Boris Island. The problem is we are trying to make it face both ways. A true hub like Dxb feeds on itself hunreds of interconnecting flights with 95% transfer traffic.
London is now missing out to this traffic , people use Ams Cdg Fra etc as it's easier to interconnect. If the 3rd r/w is built there will be yet another terminal associated with it leading to yet more complexity. I am not sure this will make travelling through Lhr any easier