it kinda makes sense, i think...
i forget the numbers, but a lot of flights into/outof Heathrow are internal flights - people who buy a flight from Manchester to New York, only to discover that they'll be flying to Heathrow along the way.
so, in principle, this new rail link could reduce the number of flights into/outof Heathrow.
leaving aside the fact that whenever I've flown from Manchester to New York it's been direct, I am aware that you can fly from Manchester to heathrow, though I would rather change planes at Amsterdam, Paris, Atlanta, anywhere but heathrow... Ignoring all that, and the fact that I don't think you can fly from Birmingham to heathrow...
Wouldn't it be much better for londoners to have the option of flying from Birmingham, which a high speed rail link from central London to Birmingham airport gives? Then the airports outside the south east can expand and take the pressure off the likes of heathrow. Adding heathrow to the high speed rail would only encourage use of heathrow.