Even if EZY were to shut STN over the winter, and that's a big if, they'd surely not drop the likes of AMS, BFS, EDI, GLA, GVA and just serve from the other end - those routes alone make up the bulk of their winter schedules from STN now anyway.
It is a bit of a strange case though. They've had very little appetit to grow at STN for many years yet they still hold 7 highly valuable first wave slots - you would've thought if there was no strategic importance of STN they would've sold them off for significant value. The problem always comes down to though where else could they expand in the south-east without further airline restructures, mergers or failures? The reality is there will be more to come and will be opportunites to hoover up additional desirable slots at LGW or LTN - but STN is still a contender for growth if required.
SEN, they seem to have found the right balance of routes but you can't ignore the added competiton from FR - I'm not sure to be honest EZY ever expected FR would show up at SEN. Is there room for both? - Answer is I think yes given capacity constraints at other London airports, but EZY rarely like to engage in hardcore price wars with FR, W6 etc. and will instead reduce/shift their network to higher yielding opportunites. Other issue for SEN is the fact that airlines are probably paying next to nothing - which is fine until such a time Stobart realise they need to invest in future facilities.