As I explained several times in the past this is not necessarily the case. An A350 and A330 would take similar development times. Both aircraft require their own 3D model and both could be benefit from having similar systems in the A380 or A320 respectively.
I've been seeing videos and it appears the A350 has many different elements in the cockpit displays. Also, the A380's MCDU is quite incomplete with many blank pages and the A350 would likely have a similar system. The A330's finished MCDU would be easier and more satisfying to use. The physical buttons are also more tactile. Also, as I've said, the sounds from an A330 with CF6's could be re-used on the 747, which has extremely dated engine sounds. The same thing could be done the other way around. Also, aren't the ECAM/PFDs/ND/MCDU the same on the 330 as the 320?
Also, even though this isn't an important thing to consider, the A330 sounds godly with any engines that it is equipped with, while the A350 sounds more like a vacuum cleaner.