Conformation is exactly what I was getting at. I have seen so few stb's that are suited to Dressage beyond Elementary level, that I could count them on one hand.
Just like how many warmbloods do you see that could do well in harness racing against stbs?
Tb's can be hit and miss. Some have excellent conformation for Dressage - I had a stunning grey with the most beautiful hock and knee action, uphill, short back that swung so naturally.
But again, the majority of racing bred tb's are not cut out for it.
Compare 100 Dressage bred WB's to 100 racing bred TB's to 100 racing bred STB's.
The warmbloods are going to win out on the Dressage ability. Sure, some won't be any good, but if they're bred in the purple, you've got about a 90% strike rate that they'll do well. If we're talking Grand Prix, it's significantly less - and in those groups of 100, maybe 2 warmbloods will Grand Prix, if you're REALLY lucky with the TB and team it up with a fantastic rider, maybe 1 will GP, and the STB's.... well you'd need a much bigger selection to chose from than 100.
Absolutely, if you love a breed and want to get that breed over doing well in a sport, then you go and get that breed.
BUT, if you want to be serious in a sport, it's best to find yourself a talented and purpose bred horse.
Just like for Cutting or Reining, you'd be far more likely to pick a QH than a WB.
It's horses for courses. Not breed prejudice, just truth. And I've competed Dressage on multiple breeds, but none have been as talented as my warmblood. Luckily I enjoy warmbloods, and have managed to select two that I get on with very well. Both are very sweet and affectionate, but talented in Dressage.