Doesn't the fact that SM completely dodged the actual Doctor's name and the fact that every answer he provides in his stories is just provides a set-up for more questions clue people in that he is the last person who is every going to provide a origin story?
Conversely, the Moffat-era Who has been marked by fairy tales and other mythical stories. While most fairy tales don't have an origin story, the story of the beginning would be one of the greatest of all — because it helps bring us to where we are now with the Doctor.
Also, the origin is ground that few have covered before and that's often where Moffat makes his hay. Bringing back a villain whose primary on-air story is lost to the ages is but one example (although I don't think the Great Intelligence was fully fleshed out in a meaningful way before the finale).
While we may not know many details about the Time War, certainly Clara would — in both her experiences with the past Doctors and her reading the freaking history of the war in the Tardis library (which I would hope she remembers at some point). It seems improbable to retread over that ground, although it remains possible if some of the hints are viewed a certain way.
In any case, I consider the Time War to be relatively small in the grand scheme of things (despite how cataclysmic it must have been to its participants). Going back to the beginning — at least for a visit — is a worthy adventure for the 50th anniversary.