Aer Lingus starting BHD-LGW was always a strange decision, it has never been an airport that has worked well for them apart from the DUB service and the token daily to NOC. The likes of SNN and ORK were tried and failed so what made them think BHD-LGW could work?! It made even less sense when they claimed the move was aimed at improving yields and tapping into the business market.
Now I don't believe for a second that LGW is the make or break route for the BHD base, I actually think that's quite an exaggeration as it's working on the assumption that EI aren't going to continue some of the summer routes year round or even add routes for winter which will require that second aircraft. A BHD base focused on LHR along with popular European routes in both winter and summer is fine, I'm sure EI would prefer BA to clear off but they are well to compete with them, what they aren't able to complete with to London is the combined effect of BA, BE and themselves (LGW vs LHR) not to mention U2 up the road.
Now I'm sure there's a few hoping that EI deeply regret moving to BHD and come running back to BFS but it's not going to happen and while I agree that LGW is a mistake, the BHD base overall may yet prove to be a wise move by EI especially if the sun routes prove as popular as early indications suggest. There is just no future in chasing LGW unless BE suddenly throw in the towel which is possible but not worth wasting too much time and money waiting for.