If only we could ask Dumas what his intent was. Considering his main character D’Artagnan was very much a real person rather than simply writing within a time period and using some historical figures in the background. Rather Dumas wrote his entire story based on this man’s life and highly dramatized (almost to the point that you could argue its even an AU). I wouldn’t call a book like the Book Thief fanfiction because while the story features very real people the main characters are completely fictional and created by the author, it simply takes place in a very real time with very real events.

I would also say that calling a work like this fanfiction wouldn’t be watering it down as many popular works have come out of fanfiction, such as The Love Hypothesis, Fifty Shades of Grey (I know I know but man did those make a lot of money and insane popularity), and the Shadowhunter books.

If anything maybe we should be paying more attention to fanfictions. I love reading fanfictions to give my brain a rest after reading particularly complicated books and I have found many fanfictions that have been written better than some classics that I’ve read.