Combat is in our nature. It's part of being a human. As individuals, we all have our own natural expression of combat aka our instincts.
The way you fight, won't be exactly the same way I fight. We may share techniques in our arsenal, but no two humans share the same 'combat arsenal'.
A true fighting system is a play-book that is expressed by the individual.
When thinking about no-rules, survival-based combat, the only thing we can do is train to be prepared for any situation, any attack, and to remove 'holes' in our fighting game.
One style does not have the answer. A style is an idea, a concept that you can use to fight. It should never be a rule-book for fighting, only a concept that might for you.