Just had a look at a site called ACI, which lists the world's busiest airports in terms of pax, movements and freight, together with growth rates. Heathrow is as follows:
Pax 64.607m (up 3.8%)
Frt 1.402m tons (up 3.4%)
Movements 466,185 (up 1.9%)
If you take a growth rate of 4% over 20 yrs, you're looking at 141m pax. So where are they all going to go?
Gatwick? Stansted? Remember, this growth rate is well below the likely rate if constraints are minimised. Air transport is expected to grow at around 5-6% over the next 20 years. If the UK's transport infrastructure is unable to deal with it, tourism, trade and (perhaps less importantly) the UK's standing as an aviation centre will suffer.
So, looking at the growth potential we have now (such as it is) and what we need, a new airport may well be the best solution. Not too easy to find a 15-20,000 acre green field site, but this may well be what is needed. It's better, to the greatest extent possible, to find a way, once and for all, for the next 20-30 or more years to find a way to provide the aviation growth the economy needs, without the perennial planning concerns, investigations and reports that seem to be required for every inch of growth now.
Time to start thinking long term . . .