Who would Delta/Virgin wish to compete with. An independent Aer Lingus or an IAG Aer Lingus.
As far as I am aware there is no cross-subsidisation within IAG, so if Delta went on the attack against Aer Lingus then they'd be standing on their own anyhow.
IAG has benefits in terms of centralisation of admin functions and purchasing, but each airline in the group has to operate on its own financial basis.
If IAG-AL was under relentless attack, what exactly do you think IAG could do about it?