I read he'd written the outlines for both the possibility of Eccleston returned and the one not.
Anyway, I don't quite believe Moffat there. Eccleston as the War Doctor? Unlikely. Unless it was the Ninth early on, and like, a day or two after his regeneration.
Why do you assume Eccleston would've replaced the War Doctor? Maybe he had an outline for a version of the story in which the War Doctor interacted with all three of his subsequent selves.
But really, if any previous Doctor were to do this, it'd be Paul McGann. Can't see why it'd have been so difficult for him to realize this - the War Doctor himself was pretty Doctor-ish in behaviour, too.
That would've had more appeal to the old-school fanbase, but a lot of the new-series audience isn't familiar with the old series, so it wouldn't have had the same impact for them.
Besides, I don't think revealing a hitherto-unknown Doctor is a bad thing. I think it was a hell of a clever twist, and I actually got quite a thrill seeing those end credits with John Hurt's name and face listed in the roster right between McGann and Eccleston. New discoveries are fun. Stories that take us somewhere unexpected are more rewarding than those that just reaffirm what we already assumed.
And while I thought McGann did a terrific job in "The Night of the Doctor," I don't think he would've worked as well in the role of the war-weary, broken-down, aged version of the Doctor as Hurt did. Having the War Doctor be elderly gave him a presence and texture that a younger man wouldn't have had, and a sharper contrast with the later Doctors. It brought something to the character that we haven't really seen since Hartnell. That's no mere stunt casting -- it's the right
casting for what the role needed. Really, I wouldn't trade Hurt's performance for anything. I'd gladly see McGann's Doctor return somewhere, somehow, but he didn't belong here.