It seems very clear to me what is happening. EasyJet is managing shareholder return as best it can. Aircraft are moved to where they provide the best return on investment.
It makes absolutely perfect sense as to why Gatwick gets priority. It's the base that is most sought after and plugging any gaps or filling any available slots is a way to keep norwegian at bay, prevent more entrants coming in or existing ones expanding. especially those with lower costs, eg Ryanair, Norwegian or Vueling. Gatwick is their biggest base it has to be protected. Sound business sense.
It is also easy to say SEN was a bad choice for them, hindsight is always 20:20 vision. But I congrsgulate easyJet for having the balls to try SEN. It is nonsense to suggest the catchment is small... It has 600k people locally, can serve London which is an enormous destination. easyJet offers bucket and spade and most popular city destinations that the UK market wants, hence it would appear the offer is what the outbound passenger wants as that is what is working. I believe that at least 80% of SEN traffic originates at SEN. The inbound market is largely untapped and this is where Stobart will spend time and money building up SEN routes at some cost. It has a vested interest to grow the airport - easyJet does not.
I think it is a good outcome that easyJet has a string core route structure ex Sen - therefore they should around. SEN is also a way of taking them out of the firing line away from FR, where fetching ticket prices is more difficult. In this way SEN is more niche and in a sense protected from FR.
A more general issue is much greater capacity in the market this year and prob same next year. To manage yield in this environment easyJet are not growing revenue significantly to its all about getting more return or as good as before from less. This explains moving of ac numbers, MAN/LPL, LTN/ STN - all about fine tune the revenue model to effect shareholder return.