I do think UA will order more 787-10s. The A350-900 is designed for long and ultra long haul routes, similar to the 787-9, but offering higher capacity. The 787-10 is a much better plane for high capacity TATL and Deep South America routes or flights approximately 12 hours or less. A bit of a waste flying planes designed for much longer routes to fly them on relatively shorter routes, hence the introduction of the 797.