FR@STN: Whilst you may consider the capacity moved to SEN brought no growth it may well have brought extra profits if they were offered a better deal. As you often bemoaned at the time, BAA probably charged top whack to airlines using STN. If SEN offered a better deal & EZY can still charge similar fares then that's a bigger piece of the pie going in to their pockets. Which I think they would consider a good deal.
Now MAG have almost certainly offered a better deal but as others have stated, EZY do not (yet) have the airframes to expand their offering at all the bases they would like. LGW slots acquired from Flybe had to be used or lost so it seems both sensible & inevitable they would fill them first.
As to business lost to FR, I think most people these days are loyal only to their wallets not a specific airline. (Timing and days of operation may also come into consideration.) If the price is right they will fly Orange again no doubt! What EZY will have to ensure is that if / when they return they manage to either pinch business back or create new business without undermining their existing offerings from SEN or LGW.