You can disagree with it all you like. But you’re disagreeing with a fact which is a silly thing to do. It’s not just a saying my German riding coach said to make us throw our hearts over fences. It’s a (personally witnessed) fact. If they know they can’t get through something they will jump it. Steel railings, brisket bars, you name it. They jump. And usually from a standstill.
If it weren’t this day and age of rabid animal rights, I’d share a video sent to me from my former boss (barn cams) showing a Holstein near the end of her lactation jumping a meter high railing with a 5’ drop because she wanted to get down into the maternity pen and away from the “riffraff” - younger cows behaving like hooligans. She cleared it, from a standstill, and landed beautifully, went to the feed bunk and dug in.
But, back to ponies. Is that downhill Quarterhorse going to put in a spectacular PSG test? No. Is it going to be able to learn all the movements and string them together in a manner that shows the maximum collection and extension for its personal conformation? Yes. Is it going to stay as sound as a horse better built for the work? Likely not. Even if it does stay sound, is it likely to show competitively beyond PSG? Almost certainly not. Would I or should anyone recommend a downhill horse of any breed for a truly competitive prospect? Nope. There are far, far too many purpose-bred horses that have a higher chance of giving their riders success at the higher levels to take the chance that a downhill horse might make it against the odds.
All the pony stuff - also from personal experience/horses I’ve personally known. 1 competitive (but maxed out at) PSG on National circuit and 1 moderately competitive GP horse on National circuit and several others that made it that far on local circuits with their ammy riders.
It can be done.