Perhaps PITAs antipathy towards SEN could be better directed at LCY - like why would anyone pay £2bn for an airport that is the countries premier in terms of diverting flights away from it! Any perceived drawbacks at SEN would, I suspect, disappear overnight if LCY was bought to be developed for Chinese real estate and closed as an airport -LCY might even be worth £2bn for that!
As for attracting new flights, all London airports (with the exception perhaps of LHR) have been subject to '3 steps forward, two backwards' during their histories as they try to build business and become established. SEN is no different in this.