If you wonder why a new Saint is taking so long it's to prevent like this happening: http://www.deadline....es-no-spinster/
There have been numerous approaches to take the Saint well away from the established character...
Are you objecting to the fact that the character's age will be different than the "classic" Marple or that she will played by an American?
We've got a fairly successful Young Bond franchise going on in print, and reboots are all the rage, showing younger versions, or origin stories of classic characters - Batman Begins, Star Trek, etc. Why can't we see an early version of Miss Marple?
And considering we've got Brits playing Superman, Batman, Spiderman, etc. - nationality changes don't seem to bother casting people at all.
I'm not saying I totally agree with it - but it boils down to a franchise's fanbase age. It's the main reason James Bond Jr. was greenlit - they were concerned that the fanbase was skewing so much older - they needed something to draw younger fans in.
Now I doubt that the Christie estate is thinking, wow, once the Jennifer Garner movie takes off, we'll have a slew of new fans and the classic books will fly off the shelves, but hey, if the check clears...