I don't know much about the 787 or A350, but I was surprised that UAL ordered equal amounts of seemingly similar aircraft. But maybe it was an astute move:

Could it be that UAL got Boeing and Airbus to bid against each other, hoping for a landfall deal of 50 jets, and lowered the price? Then, UAL locked those prices...and split the order 25-25.

I don't see it as fleet growth, just replacement. But if the lease costs are the same as the jets they replace, and maintenance/fuel burn really are reduced, it could tip the ink from red to black.