Yes that is my point. The current-generation A350 does not make sense at United unless it is planning to retire the 77W in that timeframe (which it is not). That mission, a 77W replacement for early adopters, is how a number of airlines are deploying their A350-900s and -1000s, and is probably its best application, as the 787-9/10 variants beat competing A350s in unit cost over most missions. United didn't start taking 77Ws until many were over a decade in service and those frames clearly are not in need of replacement.

Meanwhile, UA has significant economies of scale with the 777 and 787 fleets. That's why I think the current-generation A350 is a non-starter at United. A re-engined Ultrafan 350neo could be an excellent 777 successor toward the top of the capacity/range curve and I suppose United looks at that order as a good hedge if it proves to be a better solution than what Boeing can offer by the second half of the decade.