All the talk of whether AerLingus moved due to BFS not being profitable is a futile argument. They made the right decision to move based on the competitive climate of that time. Outside of the Canary Island routes to Lanzarote and Tenerife Winter sun routes were not fruitful and the non Canary ones would be hourly contested by easyJet, especially in Winter. The Canary routes were never enough to sustain a full unit across Winter, an increased flying programme was needed.
AerLingus spotted an opportunity to target bmi's frequent fliers when BA were talking over, coupled with the opportunity to move on on the LGW route. Hence, BHD presented an opportunity for EI to improve their performance. EI packaged this emergent strategy which focused on a business proposition, supplemented be seasonal sun flying programme, and having 2 routes on London have them scale on the Belfast London market.
They saw it that their cost base was sound to compete with BA for point to point traffic with some interlining. Now again the climate has changed and BA and EI will think and act as one under the parent IAG...
So all's changed, changed utterly...