Of course AA's CASM on the 772s will go down but since the 772 weighs 50K pounds more than the 333 which seats about the same amount of passengers and uses about 20% more fuel, the 777 is a suboptimal aircraft. Since AA now operates 333s, it knows that and it also knows the 777 is too much airplane for many of these routes in the winter.
So, no, AA's supposed intent was to replace 767s with 787s and it is now turning out that the 787s are being used to make the Pacific work - and pushing the 777s to markets where the long-term success rate there isn't any better than it was on the Pacific.
I didn't expect you would admit to the irony if you are able to see it.