Southend’s catchment area is even worse than Stansted.
I presume you mean because Southend has only a 180 degree catchment due to the Thames Estuary to the south? If that is the case that's too simplistic. Stansted is in the middle of some lovely Essex/Suffolk/Hertfordshire countryside where few people live. Southend's urban area stretches west to Basildon, east to Shoebury and north towards Woodham Ferrers and contains over 600,000 people. So there's a viable local catchment. Add to that Thurrock and Brentwood districts and you get another quarter of a million. Then there's Chelmsford district with its 175,000 people. And there's always eastern London, of course. Stansted's immediate catchment is far smaller. The extensive district councils of Uttlesford and Braintree, which include Stansted, amount to less than 240,000 between them. The even larger district council of East Hertfordshire can muster only 140,000.
I too have been surprised by the distances people travel to get to SEN. It's understandable that East Enders should choose SEN since Southend has always been seen as East End on Sea, but for people from Kent, Suffolk and Norfolk to travel to SEN seems slightly bizarre, yet I've met examples from all three in my infrequent visits to the airport. I still think SEN's biggest appeal will be for inward travellers from Europe once the word gets around.....