It's certainly true that IAG plays its airlines off against each other as far as investment is concerned and I suspect Aer Lingus will be having some tough conversations with IAG about its financial targets.
However, IAG's airlines are heavily dependent on each other's distribution networks and those of their alliance partners.
Vueling is, and will be even further from the end of March, codesharing extensively with BA on many routes from BCN and on its routes from UK regional airports. I suspect it will be doing the same with Aer Lingus from DUB.
It has often been claimed that one of Iberia's best performing routes is LHR-JFK, a route which it of course doesn't operate. The instant success of recent BA transatlantic route launches to San Diego and Austin is in part due to support from American Airlines, a sharp contrast to its new routes to Asia which have taken much longer to gain traction.
Distribution through AA and BA's sales channels should give Aer Lingus transatlantic a big boost and it will be interesting to see how its presence at UK regional airports will develop under IAG.