The idea that a "modern" or "classical" stance should be mutually exclusive is wrong, IMO. They are both good and appropriate in context. This is the trouble with all fighting systems, including "mma" when it is thought of as a system, their method must always be best.
I heard a pro MMA fighter say Muay Thai stance is no good, too high, weight to the back, easier to take down. This is true, but every good kick boxer knows when the weight is heavier on the front leg is the time to leg kick, (like a wrestler stands, or modern square stance.) With weight heavy to the front, it is very difficult to check a leg kick, shoulders square instead of 45 is more open, better to shoot, but worse to defend against strikes.
Squared shoulders is an aggressive posture, shoulder turned is defensive. Why wouldn't we want to train both?
I don't think it's a matter of right or wrong, if I think my opponent will shoot, I will use square stance, if I think he will kick my leg I will use Thai stance, both serve their purpose and I think modern and classical approach should be trained. (Although I think "modern square stance" is not at all new or modern.)
Gung Bu is an exaggerated square stance.
Traditional Muay Thai stance, is not unlike a high empty/cat stance.
It's not better or worse, but what fits your strategy and what your opponent is doing.