LTNman
I agree with almost all that you have just said regarding SEN, although I don't think that the slightly longer train travel time to SEN is likely to deter anyone.
What has surprised me is the large number of leisure travellers using SEN from outside of its natural catchment. I personally know of three parties from North Norfolk who have done so, the most recent having mentioned it just this week. They said they found SEN "a wonderful experience compared to Stansted" (her exact words to my wife) and liked the fact that all the facilities there were run by one entity (not completely accurate but that was their perception). From their home to SEN is a 2hr 50min drive, 40min longer than to STN, yet they still prefer it.
Perhaps easyJet are finding this is a common occurence as they have that data available, so perhaps that partly explains their continued love affair with SEN. I don't doubt they got an extremely good deal from Stobarts, but that is only any good to them if they can achieve good yields over time.