Southend cannot support either Edinburgh, Belfast - says to me that business demand is weak and/or that business in west Essex prefers London City. Can't remember exactly, but I imagine that Cityjet will continue to codeshare with KLM on the Amsterdam route at a far higher frequency than Southend could ever sustain.
The routes that remain at Southend are heavily leisure and beach focussed. This is not a good signal to send to Darwin or a network carrier. When I see an airline operate a route on sale to the public to somewhere in Germany, I'll change my mind...