FR at STN
Perhaps I should explain why STN is in the wrong place, as you seem blinded by the intensity of your own support for the place.
STN has a poor rail connection with central London, in comparison with Gatwick, Heathrow, City and Luton. There are fewer trains, and the journey takes longer. I don't think the average passenger considers the shuttles at LTN and LGW in their calculations. There is only one intermediate station, and there are no trains to other destinations other than a slow, hourly service to Birmingham (that takes over 3 hours).
Gatwick has frequent and fast trains to London, the south coast and Bedford, as well as links through Guildford to Reading. Luton has frequent trains to London (24 mins), Brighton, Bedford, and the East Midlands cities all with intermediate stops. Heathrow and City have the best links to London.
STN is on the M11, a congested motorway that links London with Cambridge and nowhere else. It only joins one other motorway, and that's the M25. Southend is similarly poorly connected by road. Heathrow is very close to the M4, M25 M40 and M3. Gatwick is less well linked to the motorway system, but Luton is close to the M1, M40 and M25. It's as close the A1(M) as STN.
The most important measure, and that used by planners, is the 1- and 2-hour road travel catchment areas. By this important measure, LHR has the largest 1 and 2 hour catchment population, and Luton is second. Due to its position in sparsely-populated east anglia, STN ranks poorly.
Where SEN scores is shorter flight times for most destinations, less congested airspace, quicker taxying times, and an uncongested terminal. It is also hugely cheaper for airlines than STN. I suspect that its catchment area is larger than STN, but have not been able to confirm this.
In the light of these well-researched facts and conclusively-argued case, I reiterate my point:
It's current role, and the only one it will have in the foreseeable future is to take the London area traffic that can't be accommodated at other, better-connected and better-situated London airports (Heathrow, Gatwick, Luton, City and even Southend).