Well, the suits have had huge success doing this sort of thing to Star Trek and Sherlock Holmes and so on, so it's no surprise they'll keep doing it. As long as low standards sell, no-talent hacks will get rich.
Sherlock Holmes may have turned the intrepid detective into an action hero, but it didn't alter his essential character. The same can be said for
Star Trek, but I'm not about to begin arguing with you over that movie again.
Turning Ms. Marple into a young detective, by contrast, is just lazy. As many others have noted, her age is essential to the character. I'd go as far as to say that it is what makes her interesting. I'm reminded of that aborted series about Dick Grayson and his family before he became Robin. He's just a kid in the circus, so, what's the point?