To take Groningen as an example, I wouldn't expect many passengers from Essex - but that's not really the target market IMHO: I'd expect the traffic to be maybe 80%+ Dutch. The attraction is not Essex (or Groningen), the attraction is that this is a convenient direct flight to London for a market that doesn't have such a service.
With no early trains from London or late trains to London Johny Foreigner will have a problem getting to and from London if the departure time from Southend is before 7:45am or arrives back after 22:30.
Each passenger through Southend generates £20 for Stobart. so 200000 passengers will generate around £4million
How do you work that one out. I doubt that Southend will charge anything for handling their own aircraft so it will cost Southend money. There are next to no shops at Southend to generate retail income which just leaves car parking and with all those Dutch who have left their cars in Holland they won't be using the car parks either.
I can only think of Manston that had its own airline to generate passengers and we all know how that ended.
I hope it all works out but I think it will be tough going.