I think Southend's chances of attracting Wizzair depend to some extent on the level of free movement post Brexit. If the UK takes a permissive line for a few years post April 2019, Wizzair will likely see stronger demand for flights and Luton will reach its 18 million capacity sooner rather than later. Conversely if the UK takes a stricter approach, Wizzair may see a decline in demand.
Southend's hope of attracting Wizzair probably depends on Luton becoming substantially full and Wizzair being forced to find runway capacity at a London airport other than Luton