Once their free ride is over easyJet will return to Stansted
ASDF, you are assuming that easyJet's presence at STN is sustainable for this to happen, and I am genuinely not sure that it is. From what I can see, the easyJet operation at STN is sustained only by the need to hold slots for nightstop aircraft at STN which preclude Ryanair and Jet2 being able to grow further. This blocker imposes limits on Ryanair's growth potential at STN and an almost total blocker on the Jet2's growth, both arising from lack of STN slots for additional STN-based aircraft.
I suspect if economic considerations alone were to prevail, easyJet would no longer have a base at STN and might perhaps serve it from AMS, BFS, EDI, GLA etc but would relocate based aircraft to SEN and LTN in equal measure. The fact that the STN base remains today is, I believe (but can't prove, before anyone asks me), testament to its blocking power which would see easyJet's overall marketshare from London eroded by growth of Ryanair and Jet2 on slots that easyJet would release if it moved out. That those slots remain a scarce resource is both a reason why easyJet won't pull what it has today out of STN but also a blocker to shifting based easyJet's own LON-based aircraft en masse from the likes of SEN to STN at the same time.