Yes - of course - London City is the only "real" London airport. But why pick on Southend (South Essex) as against Stansted (North Essex), Luton (Bedfordshire), or Gatwick (West Sussex!!!). Even Heathrow - in the London Borough of Hounslow - is arguably in Middlesex.
So - Mr. Skipness - which of the aforementioned airports (aside from City) is most convenient for the Canary Wharf / City sector?? Yes - I think that would be London Southend - no?? Should we try some comparable trips from Luton / Gatwick / Stansted? Stansted may put in a good shout - IF you want to go direct to Liverpool Street and not Docklands / Canary Wharf. With the Southend Vic. line interchange at Stratford you are bang in the middle of all the East London / City options. If you are heading for the joys of Kings Cross or Vauxhall then obviously Luton / Gatwick would win out.
You - and others - simply fail to take into account the fact that you really CAN - with hand luggage only - be on the rail platform 5 minutes after disembarking from the plane at Southend. I have done it many times!!! Yes - I know that you are then at the whim of Abellio Greater Anglia - but compare that to where you would be 5 minutes after disembarkation at Stansted / Luton / Gatwick??? Probably still in the immigration queue!!