I'm reading an interesting book, Megashift from Plot to Character In American Short Fiction : A Critical Study, 1900-1941
That's a mouthful of a title. Yes, it was a doctoral dissertation. The author is Qi Ye, and I don't know much about him, although he did earn his PhD here in Illinois.
Anyway, plot and character have both, always, been contributors to a good story, and the debate over which is more important goes back at least to Aristotle. But as Qi Ye sets out to document, there was a critical push against plot among the literary elite early in the 20th century.
Plot became a dirty word in such circles, and O. Henry, who had been so honored for his surprise endings, met with a surprise ending of his own: after his death, his surprise endings were held as a mark against him.
Of course, you should bear in mind
The reading public continued to find
His surprises delightful
And far from frightful.
Indeed, here he describes the inciting incident that led to the writing of this book:
'One surprising fact that caught my attention during my teaching of an introductory course in prose fiction at Illinois State University (Spring 1992) was that almost all the students reacted enthusiastically to O. Henry's "The Gift of the Magi," especially to the ending where it is revealed that Della and Jim have each sacrificed their most valuable possession to buy the other the best Christmas gift possible within their meager financial power, while the same students did not react so warmly to the surprise endings of William Faulkner's "A Rose for Emily" and Ernest Hemingway's "The Snows of Kilimanjaro," stories which are favored by critics.'