Too Many Dukes? Or Not Enough to Go Around?

There’s this pernicious statement that keeps popping up among authors of historical romance to the effect that In Real Life there were only a very small number of dukes and that historical romance as a genre has more dukes than ever existed in the entire universe and isn’t that just completely unrealistic?

I get a little hot under the collar every time I hear (read) someone say that because it misses the ENTIRE POINT. Which I will get to after I point out a few things.

A population explosion

Every genre of fiction is over-populated with its principal archetypes. There sure are a lot of detectives in fiction.