Heathrow getting an adequate ROI on a third runway isn't EasyJet's problem.
Airlines don't start routes just to help the airport sell more duty-frees.
LHR will want that foot traffic through its terminals and won't, I imagine risk losing that traffic by charging high fees to the regionals.
When I worked there I was staggered to hear the income generated from the shopping and it is having that captive audience of bored people with nothing to do but shop that will offer the ROI.