Expressflight,
I've never travelled, nor had any thought of travelling, with EZY thus I'm somewhat ignorant of their route structure etc. but I'm guessing that some destinations are served from LTN, different destinations served from STN, and some destinations served from both LTN and STN.
So what are they meant to do out of SEN? Any routes they move from STN (or LTN) will allow the opportunity for a competitor to fill the breach and were they to duplicate some already served destinations from LTN/STN to SEN then they'd be splitting the passengers within very similar catchment areas which, I'm estimating, would result in lower, all round, load factors.
I'd have thought the new SEN would be better off attracting close continent flag carrier regional, CRJ, AvroRJ, Fokker/Embraer regional jets etc, aircraft operations in to SEN to offer services to the close continent and for onward worldwide connections, this is the sort of traffic most UK regional airports offer so why not SEN?