I've always felt that the form of the mystery is in and of itself a literary form. In the book The Seven Basic Plots, Christopher Booker lays out all the traditional plots, giving examples from throughout history as to how each falls into one of those categories, but at the end, he found that the mystery didn't exactly fit any of those. What he realized is that it is its own new kind of plot, the first groundbreaking form in that sense in millenia. Of course, other genres break ground in other ways. And of course the loose rules of the genre are much like the rules of the sonnet. They give form and substance that the author can use to his/her advantage.
Having said that, I hate to travel back in time because I know that a lot of people don't like that, but Rex Stout was an absolute genius at breaking down the barriers between so called literary fiction and genre writing and the different forms of mystery. I love his stuff. Laurence Block is doing that so well today too.
Can't speak to Laurence Block, but Rex Stout wrote formula. Of course, people like formula. It's safe and predictable.
Nothing loose about the sonnet form. It's probably the toughest and most restricting form of any poem.
But of course it is in butting up against the edges of the sonnet that the poet is able to find his work. And I didn't say that the sonnet was loose. Of course Stout worked in formula, but I don't equate formula with not being literary. For example, one of the great things about him is that he blends the European tradition in Nero Wolfe with the America tradition in Archie Goodwin. Working within the two traditions, he was able to create something new and good.
As for safe and predictable, there were very few writers willing to take on J Edgar Hoover directly, using him as a character and then parodying him and debunking his arguments all at once. And that's just one of the many books.
Oh, well, he comes from the European tradition. Stout was doing it consciously. His idea was that the European detectives like Sherlock Holmes, Poroit, etc. were geniuses who had difficulty with everyday life. They were too smart for their own good. The tradition of the American detective is the knight who follows a code, and someone who is street smart. His idea was to put the two traditions together to make two halves of a single great man. Archie is honor, and Wolfe is genius.
Well, he's a figment of the author's imagination, but you can see the intention of creating a caricature, someone who is very different from ordinary people, an oddball, like Poirot.
I have nothing against caricature. Love Dickens' characters. I just don't like to see the paradigm of this sort of detective repeated.