Recently I returned from the Malice Domestic conference in Arlington VA. It's primarily a celebration of the traditional mystery, gathering together fans, writers, librarians and booksellers for three intense days of swapping stories and skills, and just plain having a good time.
Unlike the much larger mystery conference Bouchercon (in San Francisco this October), Malice Domestic focuses on a single genre. The characteristics of this genre have been debated often and I won't go over them again, but very broadly, a true traditional mystery (often called a cozy) features an amateur sleuth, and limited violence, sex and profanity. There is a crime, but it's often off-screen, and the culprit is caught in the end–that's the equivalent of romance's Happily Ever After ending.
Certainly it's a familiar genre–Agatha Christie is the most-often cited example, and her books are still selling more than thirty years after her death. Obviously they must have an enduring appeal, since multiple generations have enjoyed them.
But as in any genre, there are internal cycles and trends. This year at Malice I heard similar opinions voiced by two well-informed people (an agent and a multi-published writer): readers and publishers of cozies are looker for slightly rougher protagonists.

Romance Writers of America, with over 10,000 members, conducts surveys of romance readers annually and publishes their demographic analysis on their website. The representative contemporary romance reader:
–is female
–is aged 31-49
–is involved in a romantic relationship (and more likely than the general population to be married or living with a partner)
–is a book buyer, both new and used, from mass merchandisers and book superstores, independent bookstores and on-line sellers
–prefers mass market paperbacks to hardcovers or e-books
–reads at home and while traveling
There is (to my knowledge) no comparable data collection for mystery readers. I'll go out on a limb and guess that the profile for cozy readers is not much different (although for other mystery genres, there are probably more male readers). This is not a scientific analysis, after talking with a lot of readers, both on-line and in person at signings and conferences.

Or have our readers changed? There are so many demands for our attention these days, and that includes our leisure activities. Do we watch television, or movies on television? Do we make something, build something? Where does reading fit? And what does a book have to offer in order to compete with all the other attractions?
Maybe the genteel heroine just doesn't cut it any more, even as a diverting fantasy. Maybe she's just too removed from the real world to be believable.
If this is a real trend, the market will let us know, through book sales. There's nothing wrong with an amateur sleuth who gets sucked into crime-solving, wanting nothing more than to return to her ordinary and unremarkable life. But maybe women's lives have changed, and they want and expect more. In any case, a little variety can be a good thing.