It is worth remembering that while Easyjet have a base, even if it is not very big, they retain some degree of negotiating power with Luton and, to a lesser extent, Gatwick.
If Easyjet were to close their Stansted base, that negotiating power is lost. Of course, the negotiating power depends on Luton and Gatwick believing that Easyjet have the stomach for a fight against Ryanair and that Easyjet are able to make money out of having a larger base at Stansted.
Once a base is closed, it becomes much harder to restart as anybody working for a different company in aviation is that bit more cynical as to an airline's long term intentions
Senior management at Easyjet will want to think carefully about long term strategy negotiating tactics before closing a base at Stansted given the shortage of London airport capacity. I'm not ruling out, but it's a major step to take for an airline that claims to be pan-European but gains a very large amount of its profits from flying to/from London.