Yes, purebred dogs were originally crosses. But Bec, this is the 21st century, most of those purebreds are now recognized by the AKC. And you do not see too many recognized mixes in the shelter now do you? NO, because mixed breeds are just that, mixed breeds. They aren't recognized. And some most likely never will be. Cockerpoos may be because they were one of the first Poo crosses, bred a little over 30 years ago. But do you really think a Bull Boxer is going to be accepted into the AKC anytime soon? No, chances of this happening are VERY slim. Also Bec, if you pay close attention to the dogs in pounds and rescues and things, a lot of them are Poo's and Oodles.
The world is getting Oodles of Poo's these days. SO Sad.