Surely another route would be better served than DUB-LAX? With Ethiopian serving it already from June, the market is lessened.
I think the reason that entering into the DUB-LAX may take EI's fancy is that they can play two things in their favour, better connections from many points in Europe, and the pre-clearance, neither of which Ethiopian will benefit from. Plus there's a brand recognition in a sense that Ethiopian won't be the first airline that comes to mind when going USA-IRL
It's nothing new with EI, they have being building back their old core routes which are can now be operated on a lower cost base and EU connections to make them more sustainable.
I can't see EI going more than 4 weekly or even 3 weekly so there could be room for both. It really depends on how much traffic ET pick up in DUB as I suspect it should be lowish as there has to be a good deal of direct point to point from Africa-LA for them to open a route.