Firstly I don't know the answer and am someone who has spent way too long trying to get even basic Media Foundation scenarios working; at times it feels more like a quest in search of some long lost secret scripture than programming.
My suspicion is that Microsoft does want to make MF the standard media API for the Windows platform and perhaps the best indication is the Windows Universal App Samples where there are 7 instances of "mfapi.h" but zero instances of "dshow.h". Also the relatively new WPF MediaCapture and MediaElement controls do hook into the Media Foundation APIs so it seems safe to say that for all new work Microsoft are using MF instead of DirectShow.
Wow I see what you mean about Polinger's MF book! I have the book (which I'll now shortly be selling at that price) and can assure you it's definitely not worth it. I've never really found it much help despite re-reading it numerous times. It's pretty much the same as the online docs in that it sticks to a small number of specific scenarios (in fact a number of chapters in the book just re-print the SDK samples) and as soon as you need to do something outside those you are on your own.
In regards to the official MF blog there was a thread on the MSDN forums griping about thing and to which one of the Microsoft guys responded saying they have switched blogging platforms (it obviously would be nice if they could put a note indicating that). Apparently a number of the Microsoft MF devs are still blogging. This seems to be the most "active" one.
Update: If anyone happens to get drawn here due to the Polinger book reference note that the companion code can be downloaded from here (the original link quoted in the book is http://go.microsoft.com/FWLink/?Linkid=229072 which at the time of writing redirects to the aforementioned page).