Why would you want to go up against, for instance, Alicante:
At LGW with EZY, DY, ZB, BA, TOM and even FR are back from Oct '16
or LTN with EZY & ZB
When at STN there is very little competition on these 'leisure' routes with just Ryanair in this instance. Yes Ryanair are dominant at STN and have been very clever in seeing off any competition on what they view as their territory, but Jet2 especially with their holiday product, has a completely different offering to Ryanair.
We're not just talking Jet2 Holidays poaching customers from other Stansted based operators, some of these customers will be poached from the other London airports and tour operators. People will pick STN over say, LGW if the price is right. Thomas Cook, Thomson and Cosmos (do they even still exist?) are the ones that should be most concerned. Jet2 look to make a massive impact on the SE package holiday market. Their success up north speaks volumes for what they can do.