"Designer" cat breeds is an awfully vague term. Main Coons, for example, are a naturally occurring large cat from Maine. It was always a large cat, but some breeders push that to an unhealthy extreme to win cat shows. Others, like the Ragdoll, for example, is a perfectly healthy breed that started as a mix between 2 other pedigree breeds of cat with no extremes that challenge their health.

I believe that to make progress in reigning in the excesses of breeders we need to stop thinking in terms of specific breeds and start thinking in terms of genetic issues in specific lines.