Where did all the mongrels go?

[ColinM50]

Prompted by another thread, I recall that when I were a lad in North London in the early 60's every street had a roving pack of mongrels, or Heinz 57 dogs as we called them. But you never see them now. Everyone seems to have pedigree breeds or odd crosses like Cavapoo's and labradoodles, but not proper mongrels. And you never see a dog wandering the streets. So where have they gone? Just died out or been put down or turned into Chinese meals or what?