There are actually 2 other reasons at play as to why airlines move to Lhr from Lgw:
1. Higher yield. That much vaunted reason pushed as a success story actually means higher fares for us, especially at the front end of the plane. Because of this, it actually suits airport/airlines for Lgw to never become successful in long-haul. So Lhr, airlines, frequent flyer programmes, big corporations, they all act together to ensure it is always more attractive to fly from there, for a number of reasons. If you have status with an airline, are you going to miss those perks to go fly from Lgw with another airline, even though the fare might be less?
2. Airlines themselves have an obsession with Lhr, like a snobbish obsession. Think of some of the airlines that have recently moved to Lhr, like Jazeera, Rwandair, some also from Ltn and Stn and ask yourself, what benefit will it be to them? They are not part of an alliance so connecting traffic is not a part of the story. I bet the passenger who would use these airlines would use them whether at Lgw or Lhr. Think of some of the airlines who did move to Lhr and they were no bigger success there, Garuda comes to mind. Remember we all make silly decisions, so do the chief execs at the top of airlines.