People who have done force on force training will tell you that there are often two typical mistakes. The first is not moving yourself, as AK103K mentioned. By all means if taking a few step backs or to the side as you draw gives you the time you need (space permitting as well), go for it and don't become a statue. The second is an over-reliance on the pistol. Some people get so focused on the pistol being the weapon rather than themselves being the weapon that rather than make an effort to redirect the knife attack or control it and deliver blows of their own that they keep trying to draw the pistol even as they're getting stabbed with the training knife. Sometimes the better call is going to hand to hand first and then drawing the pistol if an opportunity presents itself. In any of this the ability to shoot from retention and having practiced doing that is very useful as it saves you drawing time and allows you to deliver rounds in much closer proximity to your attacker.
Even a day of basic hand to hand training can at least give you something in that event where you know you cannot draw in time. There are good instructors out there that make an effort to combine both pistol and hand to hand. None of this negates the usefulness of sights, by the way.