Wizz use Luton to swap aircraft about so they end up at their maintenance base. There are plenty of spare stands for Wizz in the morning. Every day is different but looking today there are 15 departures by 7am and only 3 Wizz arrivals. By 7:30 there will have been 22 departures excluding biz jets and 10 Wizz arrivals.
The real issue is the security lanes to get airside although the airport now opens temporary lanes via the old Silverjet terminal for a couple of hours each day which is not ideal but does the job.
In effect if Wizz opened a base at Southend they will just dilute Luton loads so will be in competition with themselves which is what Aer Arran have found out. If it is a new route they would lose the ability to swap aircraft.
Thinking about it more Easyjet operate the same routes from Stansted and Luton so I guess I am talking rubbish but I guess it all depends on total passengers per route whether a route could be split.