the word "Liebchen" that makes it so popular in the English speaking world whenever a writer wants to let a German/Austrian/Swiss character use a German phrase for "darling"?

Because here I am, contently reading a novel by Barbara Hambly I got for Christmas, and there it was again. Just as it shows up in many a fanfiction. And the thing is: I'm 42. My parents were born in 1945 and 1947 respectively. My grandparents, were they still living, would have turned 100 this year and the next two. And not only do I not recall anyone ever use the term "Liebchen" in direct address of my parents' generation, but I can't recall my grandparents (any of them) use it, either. Nor any of the people of their generation I interviewed in the mid-90s for a project, and those were quite a lot. I asked the APs whether they can recall in their childhood the term being used instead of "Liebling" or "Schatz" ("darling" or "honey"), and they can't, either. Now maybe it's a regional thing, or maybe I just went against the odds with the people I talked to, but based on personal experience: "Liebchen" as an endearment used in direct address is at least a hundred years out of date.

I recall it from 18th and early 19th century plays as an endearment, to be sure. Also it was being used in the late 40s and 50s as a deragotary, insulting term (the APs remember that much) in the third person in combination with another word - as in "X ist ein Amiliebchen" ("X is a slut for American Soldiers"). But even bigots who want to insult someone don't use it anymore. So, she asks plaintively, what is it about Liebchen that makes it so overwhelmingly popular in pro novels and fanfic alike?

After staring accusatory at my computer, a possible explanation finally dawned: doesn't one of the old emigré actors with a cameo in Casablanca use the term? Casablanca being the cult film it is, that would explain everything. Almost.

Just do me a favour, gentle reader. Should you ever be tempted to write a German-speaking character and should that character not hail from the 19th century or before, do not let her/him call anyone Liebchen. Try to avoid it for anyone not in their 70s during WWII as well. If you need convincing, Liebling, mein Herz läßt dich grüßen was a popular song from a film from 1930. Three years before Hitler came to power. Liebling. Ing. The -ing is your friend. You can't pronounce the -chen anyway. Trust the -ing. And thus endeth the German lesson, and I get back to my novel, featuring Liebchen.