There's enough people in the world who avoid LHR whenever possible (myself included), the market on this route is enormous, especially in Summer. Yes the other airport up the road is the fourth busiest in Europe, but London is the biggest city in Europe right? Also AC and BA are in the more upmarket game, the 3 in LGW are the budget carriers, so they're chasing different pax really. In my (utterly irrelevant) opinion, all 3 carriers on this route will flourish.
You make a good point. And I would love to see all three work at LGW as well as the LHR carriers. I agree, I tend to avoid LHR if I can help it, but then again I also will avoid LGW if I can. It's been several years since I've had the 'pleasure' of passing through LGW.