It's not like a full game that looks like the E3 demo exists. They most certainly decided how much they would need to downgrade before all art assets were finished.
Also there are 2 main factors that affect how the graphics end up being:
- First the hardware performance. This one everyone knows. A game is usually built around a hardware specification.
- Second is money. One of the other reasons that Watchdogs does not look like that e3 demo could be the monetary cost of maintaining that graphical fidelity consistently through an open world.
I guess they could release that demo for PC looking exactly like they promised. But whats the point?