@gbrooks9, I disagreed extensively with this idea of yours in a thread quite some time ago. So it is painful to see you blithely repeating it in “agreement” with me.
Breeders are not breeding for intervarietal reproductive compatibility. Greyhound racers are breeding for speed and do not care if their dogs can breed with chihuahuas. Shepherds in Scotland are breeding for good sheepdogs, not for the ability to have puppies with Chinese dogs.
If your idea were truly the reason that dogs can still breed with each other, we would see wolf populations that haven’t interbred since the domestication of the dog that are unable to reproduce because they’ve drifted so far apart. Instead, it is the case that the non-fox canids seem to all still be reproductively compatible: wolves, dogs, coyotes, and jackals interbreed just fine.
If you want to discuss this idea further, I suggest we start a new thread, because it is not particularly relevant to Mark’s topic.