Why would you want an extra Heathrow runway? It operates well below potential capacity. Perhaps the shortage of slots adds to their value, and perhaps protects existing operators, and perhaps justifies high charges.
Have simulations of alternative runway utilisation methodology been tried? What were the results?
Just one serious reply to your question....the densely packed airspace around Heathrow is complex and congested with multiple climbing/descending/crossing tracks; another runway just adds to the potential congestion in an area where space is at a premium.