I can tell you this: in Toronto they'll get you to re-write happy endings so they are sad or just plain dark. No happy endings in literary fiction, that is seen as the domain of pop fiction, so it doesn't surprise me the reverse is true for popular fiction. Zo and White Birds both have sad endings. Though I am going to make the ending of the third in the series upbeat no matter Toronto says. But that is the way of things.
For me, I have written enough sad endings in novels and in almost 50 short stories (not all sad endings, just enough) that I find having the freedom to write endings that come together more pleasantly something of a novelty. I'm not in any hurry to write another sad ending & have no sense of need, calling or urgency to do so. I've done enough. Nothing to prove. I'll stick with sunshine for a while.
I've written about this many times and spoken about it at writers' gatherings, even secular ones - another ingredient: religious people must be dysfunctional if they're Christian - unless they're old & toothless and then you can write benevolently about them.
When I was writing in my 20s and 30s I was iconoclastic & I wanted to show Christians there were sad and tragic occurrences in life, that not everything could be turned into Cheez Whiz with a few Bible verses - I also wanted to show non-Christians a believer could write hard and true - with White birds, even though a main character dies at the end, I made more of an effort, an effort that began with Zo, to show a fullness of life: dark and light, bitter and sweet, happy and sad - life has both - Christians can't rejoice in or understand everything that goes wrong - but non-Christians can't say there is no happiness or forgiveness or hope - even in what I'm doing with you I want that fullness - so Wings has an upbeat ending - but as you know good people die in the book from war and disease and that shows the inescapable dark shadow that sticks with us in this life - see Psalm 88, Job, Jonah's prayer, Ecclesiastes, etc. - but definitely as a young writer I reacted to Christians dolling everything up & my first novel (Mizzly Fitch)was as dark as Sherwood Anderson's Winesburg, Ohio in its way, and got me in a lot of trouble, though it's no different, really, than a text from the first part of Psalm 73 or any portion of Psalm 88. But few people know the Bible anymore except for a few favorite passages and books they continually re-read.
I think many Christians who want to write sad endings are simply trying to be honest about life experience and faithful to God in telling such tales - the difference between us is I got to do that years ago and don't feel the need to make the point anymore - but then I was not denied the opportunity as they are being denied the opportunity - but, as you say, I can't see Christian fiction becoming literary fiction so they will have to go to small publishing houses and low advances if they want to publish their tragedies - I suspect most won't do that.