No, because this very assumption that you WILL win invalidates the rest of your logic. You won't always win. In any realistic sense offense is primary over defense.
This isn't true either. There's no "general superiority of offense over defense." If you want use a different metric, like "chance to win a 1-1 fight", the calculations just become more difficult. That's why I don't do it. It's still easy to think of situations where bonuses to defensive abilities are more important than bonuses to offensive abilities.
However, I encourage you to prove me wrong: come up with a mathematical model of the chance to win a 1-1 fight (simplifications are fine!), and show that increases in offense (to hit or damage) outweigh increases in defensive abilities (hit points or defenses) unless your offense is exceptionally strong (e.g., you usually kill the monster in the first round).