I know a grade pony that has successfully competed at Grand Prix. Does that mean all grade ponies can do Grand Prix??? No.
Also, depending on which FEI judge you talk to there is a large debate on the correctness of how Fresians move WRT dressage. My friend who has shown two Fresians in PSG has gotten scores from a panel on the same test ranging 10% (from 55% to 65%, for example), sometimes more (!) because of the wide disagreement in dressage circles about their movement.
Warmbloods also are NOT just a mixture of hot and cold blood. It's not like water. They were developed from "the best" riding horses in Europe with Arabian and Thoroughbred blood intermmitently added by the studbook to refine and create a more athletic horse. While at the very beginning of some breeds, work horses were used as foundation stock, they were quickly refined with cavalry and riding stock.
You cannot cross a draft horse with a TB and call it a warmblood. Again with the thousands of years of breeding argument. For thousands of years, drafts were bred to pull. For thousands of years, TBs were bred to race. Crossing them does not magically make a dressage or jumping horse, or a "warmblood" for that matter. Warmbloods themselves have also been bred and refined for thousands of years for the purpose of riding and sport.
Piaffe in the field is rarely correct, even by a warmblood. It is usually prancing, out behind and while that's awesome your horse can prance like 99.9% of all other horses in the world, it still does not prove that drafts are born and destined for the international dressage ring.
Just like how 99.9% of horses in the world can prance, 99.9% can also perform working paces undersaddle and do a lower level dressage test.
They say money doesn't buy happiness -- well happiness doesn't buy horses!