Now this may not be the case, but my interpretation of Shinobi's post is that the E3 demo is basically just a vertical slice which will bear little resemblence to the actual sequence in the final product. The demo's purpose was to tease what the story will be about and how it will unfold, along with introducing a brand new side of Kratos, the kid and revamped combat mechanics. With the demo behind them, SSM are now pretty hard at work, building up the game. Now, I wouldn't rule out a Fall 2017 release yet, but a delay to 2018 is very likely.
Edit: Again, I would like to point to the E3 2012 demo of The Last Of Us. You can clearly see that many instances had highly scripted AI, where they performed perfectly on time to keep the flow of the demo. In addition, features like crouch running and hunters looking in drawers were completely removed and the UI was redesigned completely. The demo is basically a showcase of the gameplay mechanics and tone of the game. Yet, TLOU released a year later. I expect the same for GOW4, well, except the release date.