The need we have to whizz all over the place to talk to one another has been obviated by video conferencing et al. Business and government haven't woken up to this yet- but they will.
Air travel is so unpleasant these days- for all sorts of reasons- it's a wonder people still put themselves through it. The next generation- never having acquired the habit- will have better sense.
Brexit is already causing people to take their holidays at home instead of Magaluf or Phuket. As we get poorer- as we almost certainly will as an unsustainable global economy goes pear-shaped- we'll be globetrotting less and less.
Opposition to the expansion at Heathrow is powerful- and has champions in high places. Already the decision (still warm from the oven) has triggered a by-election. It's an issue that could wreck this divided government with its very small majority.
There you are: Five excellent reasons why the third runway at Heathrow will almost certainly never be built.