I've seen so much in all these years, and im up for more surprises. I've seen Standardbreds outjump warmbloods, I've seen a Hanoverian win in western pleasure and trail against quarters and paints, Arabians cut, Haflingers beat quarters in open reining, clydesdales do western and dressage, Friesians do western, there is no limit. Good riding is good riding, and a good horse is a good horse.
Nobody should stop anybody from doing something to grow.
And I can see Western dressage as an alternative for all those western riders who don't have intentions to show pleasure, or reining, or any of the other disciplines offered in breed shows.
I don't think there is any danger for the existing competitions at all. It should be fun, not drama.