I think you'd be amazed at how fast LHR might be closed down
Well clearly that wouldn't happen, any more than Manchester was closed down after the Argonaut came down in the middle of Stockport, or Schiphol after the El Al 747 went into the apartment block.
What would change, of course, would be the dynamics of the Heathrow expansion argument - but we're talking about a hypothetical event so unlikely that there seems little point in considering it.