I am always surprised at the number of connections onto BA domestic and European flights from Heathrow that come from non-OneWorld carriers, there are obviously plenty of off-alliance through fares that are available (as an example, the recent medical case in the press of the nurse who returned from West Africa to Glasgow, Royal Air Maroc to Heathrow and then a BA domestic leg). This routing is just not available through Gatwick.
Sure Gatwick has done well again for passengers, I suspect it's almost entirely driven by the shell-suit lot on Easyjet and Norwegian. Good for them. And Gatwick has managed more than half the number of passengers at Heathrow while only having half the runways, and a fraction of the widebodies. But yes, if a third runway were opened today at Heathrow both BA and Virgin, even for their so-called "leisure" destinations, would be over there tomorrow. There's a very large proportion of higher-fare business traveller, in particular inbound from overseas, who just look for a Heathrow service - want a choice of half a dozen daily departures back home, high-tech customer in the Thames Valley, taxi/chauffeur service to Central London taking less than half a day, etc.