Sorry but this just isn't true. It is quicker if your points of origin and destination are near the terminating station. Lots of businesses are closer to the airports. Usually it is much cheaper to fly too. As a regular "by air" fan, I did try the train a few weeks ago (on a Sunday). Never again. At least on the plane you are guaranteed a seat and it is possible to buy food and drink and walk to the lavatory. Try the train on a Sunday (even F class) and compare. Even the nightmare that often is MAN T3 security compared well.
You were lucky! Getting a train on a sunday on our 5 days/week railway is an achievement.
Yes you're right, unless you're going city centre to city centre on a weekday, the train isn't quicker.
Unlike Ringway, trains from the north do NOT run to Heathrow.
Maybe when HS2 turns up that situation might change but until then via LHR remains important.
More like "if". However, if that monumental waste of taxpayers' money does turn up, even more economic activity will be sucked into London and the "Northern Powerhouse" idea will be history.