That and the Reach would probably marry Marge to F!Aegon so they could still have her be queen.
I'm thinking we need to tell Stannis the practical thing to do would be to combine our armies, leave some of each to besiege King's Landing and make sure it ain't going anywhere, march up and destroy F!Aegon, then have a peace for two months before the fighting starts again. And if we're lucky one or both of our two Baratheon kings dies in the field of battle and we don't have to care about the question anymore.
EDIT - Oh, and Dorne is probably about to join the war too.