Originally Posted by UA_Flyer

The jury is still out on that engine. UA is not going to commit to new engine type without proper performance and reliability track records.

One of the main reasons amongst a number of other reasons to retire 744 was the availability of parts and maintenance capability and capacity around the globe.

If UA order 788s with Trent 1000-Ten, will there be spare parts and maintenance expertise at destinations if repairs are needed?
Use the same logic for 777X, so few airlines are planning to operate the 777X, will there be spare parts and maintenance capacity at places UA fly to?

A350 is a very popular plane and have been and will be flown by many *A airlines. Spare parts and maintenance will not be an issue. I personally love flying the a350s. More roomy and quiet than 787s.

I understand OP's interest in the 777X, but there are a lot of things have to happen such as cancelling A350 order, order more 788 with an unproven engine type, and then introduce another new plane with another engine type, etc. Why would UA go to all that trouble to order 777X that may only serve a handful of markets.