This question becomes speculative when considering all aspects of the marketplace including. The price of each aircraft as the main driver for this class and followed its advanced technological efficiency. Passengers become an afterthought. The 787 has captured the standard with sure numbers of ordered and delivered 787's. The big view gives a better look at what could be expected. During 2013 both the A350-1000 and 787-10 made an appearance with its respective first orders. Today the order count for both types are at a deadlock with a net 168 A-350-1000's ordered and Boeing's 171 787-10 ordered. There is a subterfuge order for forty more 787-10's with Emirates yet to be confirmed with a signature as negotiations move to a closure.
The above chart suggests an order trend more than an answer for the A-350's future. During 2013 was Airbus last good order year when the A-350-1000 was first introduced with bulk orders. The Airbus order years for the A350 during 2014, 2015 and currently 2018 were disasters where cancelations rue the day. The overall recap of a five year period. Boeing leads Airbus by a wide margin with more 787's awaiting a final confirmation as mentioned before. Boeing stands at 1,365 orders while Airbus stands at 832 ordered the type within respective classes of 787 and A-350. But the real market is the delivered units which Boeing has and will not relinquish to Airbus within the next decade.
Airlines have assimilated its 787's in a massive and effective manner setting the airline industry towards the 787 as the leader. Airbus will take twice as long to reach Boeing's current pinnacle in the marketplace. The A-350 could just die from inability to answer the 777X, 787, (797) charge. It may just recede into a single aisle master similar to what Bombardier and Embraer have done with its regional single Aisle offerings. Bold prediction, but entirely possible if Boeing achieves permanent dominance with its extensive widebody offerings.
The Airbus widebody effort is dying from the A-330 to the A-380 family of aircraft line-up and the A-350 is dead center in Boeing's Bulls Eye.