Not meaning to sound like a stocker, but where do you live (don't tell me what town, just the state or region). I think a lot of it has to do with the area you are in. I'm actually really new to Dressage and haven't shown in it yet, but I will this coming year. I haven't been around Dressage long enough to see what is popular and what's not.
I'm slightly more inclined towards the larger cob type ponies, though you don't see them a lot in my area. They are just so versatile and hardy. Many can even carry full grown adults, they are my favorites to ride. They don't really relate to this topic though because really I would have no trouble at all training a pony like that, they are just my perfect size. But still, people do still want and need ponies, and I think that if I could really train GOOD ponies and work to promote them I wouldn't have trouble selling them.