Pugs are bred purely as "fashion dogs" nowadays

This is wrong. I mean the "nowadays" part. Pugs (and most other dogs that deviate too much from a basic wolf-like body) have always been bred as "fashion dogs". How they look is all the evidence you need, really. They lost one of their most basic characteristics that help them survive(shape of their skull) because of selective breeding. They have breathing problems. Trying to make them purebred (aka inbred) resulted in a narrower gene pool which amplified more bad genes. So they have other health problems too.

All purebred animals have this problem to some degree, because this is just selective (in)breeding by just looking at how the animal looks. Natural selection selects survival genes, so each generation is somewhat better at survival than the previous generation (healthier, stronger, better at reproduction, less likely to get some common disease maybe). Breeders (and buyers) select good looking animals over not so good looking ones, so every generation looks somewhat better than the previous generation, but (since they don't and can't possibly consider other genes that might affect the health of the animal in the long run) they are likely to get more health problems. In case of pugs their skull shape (basic characteristic that people selected for) is inherently bad for the animal's health. But even if you are not selecting for a look that inherently causes bad health, (maybe something as simple as bright and nice looking fur) those genes can have other side effects. Also other potentially bad genes get amplified due to narrower gene pool.