What not building the runway will achieve is making the UK less connected and less competitive, stunting economic growth and costing jobs.
Will it? About a third of Heathrow's passengers are international transfer passengers and so, other than buying an overpriced bottle of water at WH Smith, contribute very little to the UK economy. And they, generally, only transfer through Heathrow because BA price the fares competitively; this is why it's cheaper to fly Dublin-Heathrow-New York than Heathrow-New York. Good for BA, no doubt, but as IAG are Spanish again I'm struggling to see a benefit for the UK. A further runway will reduce BA dominance at Heathrow, of course, but this won't change Heathrow's airlines using it as a transfer hub.
As for "eco-loons", well, it's amazing how anyone who disagrees with anythimng is either a "loon" or a NIMBY.
Me? I think expansion for expansion's sake is pointless, especially as the beneficiaries don't and won't contribute to the UK economy. There is plenty of spare capacity elsewhere in the UK, but airlines (other than Emirates, bizarrely) don't want to use it. There's nothing to suggest that transferring at Heathrow rather than, say, Schiphol, will add any benefit to the UK (indeed the opposite seems to be true- KLM Air France and Emirates do more for regional connectivity than British Airways ever have!)
But even more importantly I think Heathrow is the wrong place for expansion; the final approach over densely populated central London is already a disaster waiting to happen, never mind adding more and more to the mix.