SEN is for me an example of how good planning and a convenient train station can attract new airlines and passengers.
For many reasons despite the Stobart investment, a good passenger experience and a convenient station the airport is going to struggle big time when there is capacity at London’s other airports. It is the furthest away from Central London (but only by a mile compared with Stansted) has the longest travel time and doesn’t have a direct link to the UK motorway network. Southend will flourish again but only when the other airports run out of slots and when airlines feel they have a need for SEN due to a lack of alternatives.
I note Eastern Airways new Teeside service to Heathrow. That would not have happened 6 months ago and could have been a natural route for Southend when other airports could not offer appropriate slots.