Surely you're conflating capacity growth with profitability, too much capacity isn't what a highly seasonal operation like DUB-US needs, hence the B757s being introduced.
Also, I think it's fair to say that just because the Irish Gov has no direct stake in Aer Lingus, it's a long way from encouraging a Middle Eastern airline to aggresivley target an Irish based company, the de facto flag carrier for want of a better expression.
Wasn't there a fuss at the time when Airbus tweaked the A350 design to the XWB that this was a little too capable for what EI was after?