Before I start this, I just want to clear up that this is all my opinions and I have no actual knowledge of A350 plans past what has already been publicly said. But here are the few reasons I believe the A350 won’t be released anytime soon, even after the aircraft already planned will be released.
It is a new plane.
The A350 is the newest plane currently in commercial service, and while the A330neo isn’t far away, the A350 isn’t based off of a past model. A lot of performance data still isn’t publicly available, and there are still variants in development (A350-900 upgrades, A350-900R and ULR, and the possible A350-1000ULR).
There is already a plane in IF that serves its market.
The 787 is built to serve the market the A350-900 was built for, and the 777 is built for the -1000’s market. While they are very different planes in terms of the way they fly and the way they look, you can already fly a ton of routes that will be/are served by the A350 by just spawning in the generic livery in a 787 or 777.
There are plenty of improvements to be done to current aircraft.
The simple fact is, the game has a lot of aircraft that were developed before they had access to technology that could accurately simulate ground effect, wing flex, cabin lights, and things that still have yet to be added such as working cockpit instruments, truck (gear) tilt, and landing smoke. At least in my opinion, their time would be better spent adding to things that are already in the game to make the overall flying experience better.
While I do love the A350 as an airplane (and I hate to admit that as a Boeing fan) and would love to have it available to fly, I would rather see current game features updated than have a new plane added. I would also like to see people stop asking for a plane they won’t get, and I’m sure the Devs would as well