Yes, Eccleston was always the original choice for the role of the War Doctor, but he turned it down, hence some storyboards near to the final script still showing the Ninth Doctor with the Tenth and Eleventh.
Very shortly afterwards, John Hurt was cast and had to say yes very quickly. The idea wasn't exactly a last-minute one, but the casting may well have been. Moffat said that Eccleston turned down the role because it's not the sort of thing he does (i.e. return to a role), although there is evidence to the contrary (series 2 of Cracker and expressing an interest in returning to Heroes or Our Friends in the North).
It could've actually been for another reason, such a scheduling clashes (he was, I believe, still filming some of Thor 2) or, what I believe to be the reason: that he didn't want to play the War Doctor role if it would undo everything that his character of the Ninth Doctor had done in series 1 (2005).
For those who believe that the Eighth Doctor couldn't have pressed the button, let's not forget that if we are counting televised adventures only up to the beginning of 2013, McGann had only one, and most importantly, it was a regeneration story. In this story, the character hasn't even fully formed yet. People seem to base his "dashing, romantic" persona on that one appearance, which they shouldn't. Countless audio dramas have darkened his role; something which McGann always wanted to do in the first place.
The way I would've actually written it would be that McGann, Eccleston, Tennant and Smith would all be there to think about pressing the button, but McGann suddenly pushes it. He explains that because the Moment is psychologically linked with whoever possesses it, it can take on the form of whatever weapon the possessor wants. In the Doctor's case, pushing the Moment hides Gallifrey in an instant and lets the Daleks wipe themselves out. From there, the Doctors return to the art gallery and McGann goes off in his TARDIS, only to crash land on Earth, 2005, where he regenerates in Eccleston, finds a leather jacket at Henrik's and goes off to find some Autons. I wrote a full synopsis here:
http://gluben.tumblr.com/post/117182...-shouldve-gone
Imagine now if Eccleston had been asked to reprise the Ninth Doctor role but in a completely different capacity, with a separate storyline that links him to the others. Would he have accepted then? I'd like to think so, so long as he was given enough time to read a script. This is something else which probably stopped him returning, and even Tennant is on record as saying that he was annoyed to be kept waiting to the very last minute to be informed of being in the 50th (Eccleston obviously could not afford to do so). What if RTD and Moffat had co-written it, and decided the storyline a lot earlier, say, back in 2010? That should've been how it happened.
There was also a university event which McGann attended and questions were put to him by the audience and others. I wasn't there, but I asked the question of would he like to have played the role:
https://livelaughreadwrite.wordpress...t-by/#comments
His response:
"I do remember though that Paul said of course he’d have loved to have played the part of the War Doctor if it had worked out that way, but they got John Hurt in and he’s a brilliant actor and that’s what happened. Think he did comment as well on the fact that it was slightly unfortunate that there was no time to connect as such with the War Doctor because he was completely brand new to the show."
So I do think McGann should've been asked. It would've been a perfect link between the old and new series and provide both closure to his Eighth Doctor and a mirror image of the 1996 TV movie. That's the point really, that the Time War could've changed him into a much darker character, only to be redeemed at the end.
And for all of that, I think the 50th really killed off my last feelings of love for the show. The split series 6 was the start, but that was the straw that broke the camel's back.