The Emerald/BA Dublin flights actually connect well to BA long haul. Do you really think they would not have examined the economics before operating the LGW to Dublin the route?
I wouldn't think too deeply about it. They haven't. The reason why this is Emerald for BA (and not Aer Lingus) is that it's a last minute operation cobbled together to cover some BA Gatwick slots. There's nothing strategic about it whatsoever, and if this was part of some grand master plan as to how to serve hubs, this service would have been unveiled as an Aer Lingus franchise route when Aer Lingus itself withdrew, with whatever forward bookings between Dublin and Gatwick they had transferred to the new ATR service. As it is, all of those customers were re-routed and now they're starting a new twice-daily service at a month's lead time with no advance bookings.