In a truly capitalistic society prices are driven down by more choice, if more space was available at LGW then the likes of Norwegian I believe would offer more US flights at low prices and you would have EasyJet acting as their feed.
This in turn would lead to BA competing with lower fares to stop a drift of PAX to LGW. New York has 4 airports spaced around the city all fiercely competing,
This would be a much better model than putting all your eggs in one basket.