the fact of the matter is that airlines only fly from LGW because they can't fly from LHR
That is an exageration, only about 45% of Gatwick traffic would gravitate to Heathrow and only if the appropriate service types and fare levels were made available there. easyJet clearly recognise this and I've always reckoned Norwegian would grab some of the Heathrow action as well to properly take on SAS if they ever got the chance.
This still leaves a substantial market for Gatwick drawing from South London to the South Coast - something similar in size to that of Stansted.