Well obviously natural selection would kill off several breeds, it's why when you see street dogs in other countries, most have short hair, long snouts. I'm not saying a pack of pugs would be able to survive in the wild per se. Also, you said the domestic dog would be essentially helpless in the wild. The domestic dog is still a vicious predator, which, if free from human intervention, can again regain it's ability to function in the wild. My point being, at some point, dingos, African wild dogs, and others were domesticated dogs who have adapted to the wild over the centuries now. You mean to tell me tomorrow, without humans, the thousands of huskies in Alaska wouldn't become feral and wild and start hunting and forming packs again? Just read up on feral dogs, various packs that act very much like their wolf ancestors.