I'm sure that if Southend's runway was 500m longer (which it can't be) they would have no trouble attracting airlines. With the A320/321 and B737-800 so ubiquitous among European airlines there are many carriers that could not consider using SEN because of the restricted runway.
BUT..... there should still be enough companies with aircraft that can operate efficiently from Southend, AND we need to wait and see what the A320neo can do in terms of take off and landing distances.