Position is but a subset under a heading that I would title "tactics", which ultimately fall under "strategy".
Wars, battles, gunfights, and fistfights have been fought over position. A former Ranger friend of mine had a saying, "Terrain dictates." That statement could easily be "Position dictates." Position should determine what tactics are being used. In fact, in my mind, tactics and position are almost interchangeable, but changing position should definitely fall under tactics.
The fact is in a real street fight things happen so fast that all a person usually has to fall back on is the training, and as Hock has pointed out, if the training emphasizes position over tactics, this is what will generally come out during a fight. Strategy drives tactics, and tactics should, in my opinion, drive position. If all a person knows is boxing or kickboxing, then that person must use those tactics to drive their position. Likewise for someone who is a wrestler/grappler.
And then, of course, what to do when the other person compromises your position? If you're in an ambush and your security gets rolled up, how do you respond? If you're the one ambushed walking down an alley, and your kickboxing position is instantly compromised, what do you do? One must not rely on position, but must rely on tactics, and ultimately, strategy. Know what you're going to do in a situation, and have a plan. If you get rushed as you pass a corner, what will your response be? Roll with it and try to pull the person into your guard, or achieve the top mount? Or try to keep your feet, clinch, and give them a few poundings? Will you break contact or become decisively engaged? What's your strategy and what tactics will you use? How will you achieve these tactics if you're in a compromised position?
These are the questions that I feel are rarely answered by the average classroom instructors, to include the Army Combatives instructors.
Kent