Ryanair + Aer Lingus would have better prospects for traffic at DUB than IAG + Aer Lingus and why wouldn't FR want to tap into transatlantic business. They probably provide the vast majority of PAX transiting through DUB on their way to the US as it is.
WW has already stated he doubts LHR will ever have a third runway so expansion there is capped and slots cherished. Slots are why he needs EI.