OK, I think I know what they are going for here.
They're wrong, but I can see it.
See, in my mind, the modern conception of Miss Marple would be someone like Helen Mirren or Dame Judy Dench.
I know they want to work with an established property, but I think they should just do the show they want to do and keep Miss Marple out of it, or at worst, make it so that the original Miss Marple is Garner's slightly distant relative, like the sister of her grandfather, so she can have the name, but she is a completely different person.
I would respect that a lot more.