Shannon needs services to LHR to serve the West, because it's a vital link for business and leisure.
But it's not a vital link for the airline unless it can make money out of flying the route. Welcome to the world of reality. Airlines have to do everything they can to maximise profitability and they are not social or charitable undertakings. If Shannon needs a service to LHR so badly, either the Government needs to apply for this to be a PSO route and offer subsidies to ensure that it is operated; or it shouldn't have sold its controlling interest in Aer Lingus to start with so that it could exercise sway over Aer Lingus' management decisions if it wished. Looks like it's a bit late for either, so there isn't really much option but to move on and get on with life.