This isn't true. Suppose that you're trying to minimize the average damage you'll take from a monster before killing it (and we assume you'll always kill it before it kills you!), and it gets to go first (for simplicity).
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. Only the very most simplistic case of an "offense bonus" and a "defense bonus" which are exactly symmetrical and where N damage taken by you is as good as N damage delivered yields the result that defense and offense are equally good. 4e violates these conditions at every single turn.
There is a more subtle reason why offense trumps defense as well, which is the general tactical argument. A high offense force can deploy itself more easily to greater advantage, take better advantage of mobility, probably gains more advantage from terrain, etc.
In practically every sense defense is inferior to offense both as a general tactical principle and specifically in the context of 4e combat.
Now, that doesn't mean it can't be even more advantageous to have the tactical flexibility of say having one very high defense unit and a number of high offense units to back it up. However this will depend on the mix of tactical situations you face and the skill of the opposing commanders, as well as factors like how spread out the units are relative to their effective engagement ranges, etc.
SOD-like, or even just the common generally degrading 4e conditions, effects also militate more towards offense. You want to DEPLOY these effects and disable the enemy quickly vs hoping to resist his employment of them. Again this mostly devolves down to tactical advantage. When you get yourself into a good position you want to strike a decisive blow, not nibble a bit on the other guy and then let him react. His first mistake should be his last mistake.
In any case my point stands. Defense is significantly less valuable than offense and in 4e offensive boosts are actually cheaper than defensive ones. Its no wonder that in general by epic tier most characters are doing crushing amounts of damage and hitting on low numbers yet their opponents hit them in return on equally low numbers, usually. There isn't exact symmetry between PCs and monsters of course, but close enough to draw reasonable conclusions which I've validated pretty well in actual play.