But you've just lost more than half of Gatwick's catchment population, and the same goes for Luton. You can't just 'work with East London, Kent and Essex'. There isn't the demand which is why Southend has barely any flights.You don't necessarily need to get Gatwick's core catchment in Sussex and outer South London, you could work with East London, Essex and North Kent.
It depends where in the home counties - Kent, Essex, East London, eastern Herts could be easily reachable for Southend.
No airline is going to move if that's the consequence as it would have a huge effect on their revenue. Airlines, particularly LCCs, ideally base themselves where catchment populations are maximised with as little overlap as feasible, hence Jet 2 having bases at Newcastle, East Mids, Leeds/Bradford but no interest in Teeside, Humberside and DSA as just one example.