Wild Goose,
"Devx
The A350 is not a competitor of the 777-X.
The 787-9 competes with the A350-900 and the 787-10 with the A350-1000"
Those two statements are actually incorrect.
Airbus does tout he A350-1000 as a competitor to the 777-9X on a seat cost basis despite its smaller size (carbon vs aluminum fuselage). The real question is whether the -1000 will have the range to compete with the 777X. Airbus says it will, but there are reasons to be skeptical (namely underpowered engines). There is no doubt the 777-8X will have greater payload range than the -1000 (same size fuselage), but some believe the -8X economics will not be good as the -1000's.
The A350-900 is actually of similar size to the 787-10. There is little doubt that the -900 will have more range than the -10, but the economics for the 787-10 look significantly better on missions less than approximately 6000nm.
The 787-9 is the competitor to the A350-800 size-wise. Both are to be ultra-long haul aircraft, but the A358 is rumored to be scrapped because it is much heavier than the 787-9 and far behind in the development phase.
So in sum, the A350-900 is the only member of the XWB family that has no direct competitor, but that is only the case for missions over 6000nm.
The A350-1000 seems to be in a state of flux as the final design appears somewhat uncertain at this point. There are rumors that Airbus will actually scrap the -1000 and build a "-1100" to compete more directly with the 777-9X. Either way, I think it is fairly certain to say that Airbus is definitely reacting to Boeing widebody products, and not the other way around. (The opposite case could be made wrt to A320NEO vs B737MAX.)