FRatSTN
I'm sure it was just a typo on your part but easyJet started at SEN in April 2012, not 2011.
You are quite correct in listing all the elements that had to be considered before the decision was made to establish a base at SEN and your frustration that STN was the loser as a result is very understandable. The case for doing so obviously convinced the Board and the addition of a 4th based aircraft in 2013 seemed to be a success. So far so good but then things changed at LGW when they purchased the Flybe slots and at the same time they were facing potential competition from the likes of Norwegian, so an increase in based aircraft there was the result and SEN lost one aircraft for 2014.
I don't see any of the above as necessarily meaning a further decline in the SEN operation going forward. All airlines adjust their strategy to reflect market conditions and the actions of competitors etc. and this is just another case of such an adjustment. The size of the future easyJet operation at SEN may increase, may stay the same or may reduce further, who knows? One certainty is that the potential for further growth in easyJet passenger numbers still exists at SEN and the terminal facilities now in place certainly make it an attractive option to passengers from a catchment area of at least 2 million people.