Well, I guess we have proven it IS possible (we can document it off course), although parts of ours is also "external" like the engine model and some other parts that are "modified". We spent about 6 years on our flight model, so it has not been a trivial task - but worth it. They choosing a different route is off course up to them, but just because they did not succeed does not mean that others can't! Not sure what they did, but I suspect it is highly table based since from what I understand it is ported from some other sim, and table based flight models have their share of weaknesses. In a way you could say we (IXEG) takes the best from both worlds. Also, on an A320 - that uses FBW (artificial stability and augmentation) - that you hardly ever fly in direct law, the dynamics are not as important from pilot or engineering perspective anyway as it is on a classic Boeing without FBW where you fly hands-on in "direct law" a lot of the time.
But I wish them good luck, a great A320 finally would benefit XP a lot and attract more users from the other sim's
M