Is it going to (eventually) replace for MDX2.0 ?
XNA, for all intents and purposes is MDX2.0
... XNA/WPF questions ...If you're looking to build a desktop application that incorporates 3D elements, then WPF is definitely the way to go.
If you're looking to build an Editor for an XNA application which reuses code (say, reusing the sprite rendering engine for your game in a level editor), then you can render to a control just as you could with MDX. In fact, I've written a control that you can drag and drop onto a form to render with XNA:
Hope that helps