I guess I'm not understanding the problem. I didn't pick up that anyone was saying you could not use XYZ horse in dressage. All they were saying was that to reach the highest levels of that sport, you'd be better off going with breeds that were designed for it. You might get an exception to the rule. It does happen. But if your plan is to do it big, you might find the path of least resistance a much easier road to follow.
If I'm going to be going out cutting cattle I'll be looking for that athletic, quick, low to the ground Quarter Horse that can get low, dip and dive and turn on a dime. I could do this on a different breed for sure. But will it be as graceful or done as well? Probably not. Exceptions to the rule not withstanding of course. :)
And just for fun ... this was one of my favorites to watch. May she RIP.