This debate all comes back to the fact that STN is in the wrong place, as I have said on this forum frequently
It serves few nearby towns of note (there is Harlow, but Bishops Stortford & Dunmow are small market towns), whereas SEN serves Southend itself, Leigh/Basildon/Rayleigh, Canvey Island, Billericay, Grays & Brentwood. Luton is in close proximity to Luton/Dunstable, Hitchin/Letchworth/Baldock, Stevenage, Milton Keynes, Harpenden & St Albans.
The debate about rail access is a bit of a side-show too. Most travellers (over 50% in all cases) arrive by car. SEN scores by having parking close to the terminal, whereas at STN, long stay parking is at a significant distance from the terminal, and expensive. That makes a real difference to travellers, and those in the know will choose airports with cheap parking close to the terminal.
EZY are quite correctly acting in the interests of their shareholders, and if they can operate with similar yields from an airport with significantly lower costs, less ground and airspace congestion and good access to 'airways', they are right to do so.