No-one has picked up on SOE's point that SEN is "-on-sea", meaning it has a limited catchment area.
Draw a 30 mile radius circle centred on any airport, and generally you've got it's core carchment area. If half, or a third, of that circle is sea, then few people will live there and that will reduce the airport's catchment area. If some of that circle contains a wide estuary with no direct crossing, land on the opposite bank will not be core catchment area either.
Southend suffers from both effects; few people live east of the airport and North Kent's access to the airport is convoluted, to say the least.
I wish SEN every success, but I don't expect it to ever be more than a local airport.