My understanding is that A350s are long haul aircraft. So I don't think CX will look to replace regional 777 or A330 with them. It seems to me that CX will have to choose between A339 and 787-10 if they want a true regional replacement. They may choose to intentionally misuse long haul aircraft on regional routes as they do now with A340, 744, and 77W; but I don't think that replaces the need for a regional fleet.
There are indeed more A333s coming. That would lend one to believe that A339 is the logical successor. However, A339 only makes fuel over A333 on routes above approximately 3 hours due to increased weight from the bigger fanned engines and wingtip extensions. So, I don't see the NEO being a slam dunk by any means.
You brought up the aging fleet of 772/3s at CX. Clearly, there is still a place for larger regional aircraft. Note all the 747s flying regionally. So, to argue that the 787-10 is too big does not make sense.
If CX were to buy 787-10, then it does open the door for 787-9 for new, long, thin routes beneath the A359's size. I'm not saying it will happen. I'm saying it is a possibility. This might be more necessary if US and Euro carriers start flying 789 from medium size cities to HKG (ie Norweigan Air).