Its possible too that private breeders/fish farms can also sell you culled & sickly fish, they do not only come from pet stores. Personally, I worked for private pet store for 3 years many years ago & they do lose money when fish shipments dies in the store, because some suppliers only credits/refunds for DOA fish. So, it wouldn’t benefit them to kill fish their trying to sell & make a profit on they’ve only had a week or two in the store. The store is losing money.
Personally, I had the opportunity to learn how to run a store & a ton about the hobby, saltwater & freshwater in that environment. Maybe Private people are also breeding fish to make a profit as well, and may sell overpriced sickly fish to people.
Personally, if I wanted the best of the best discus, I’d go directly to the source & order it directly internationally from suppliers of your preference. You can purchase fish directly from wherever they come from, I’ve done it before & it’s a process, but it can be done. I learned how to from the pet store I worked at ordering some wilds or directly from the suppliers southern America or overseas.