And bmi said it wasn't a profitable route
If there had been no head office cost or a significantly lower costs one (ie withe scale of BA operation) the route certainly would have been profitable.
It would have appeared that the costs of the airline meant that all seats would need to attract fares too high than were realistic, hence they were collecting less revenue than they needed to break even or be profitable, I would imagine most routes were not a lot different to Dublin and lets no forget Aer Lingus is competitive when compared to bmi and the have much greater capacity on the route.
In addition, close to the time when BA axed Dublin Heathrow in 1991 they had often L1011s and 757 on daily schedules. I expect they will do fine on Dublin/Heathrow aircraft type is not always an indication of profitability. As someone elsehas posted a bigger plane may take the place of two smallers ones.
EI-BUD