I'm going to play the devil's advocate and argue that, from a game design stand-point, having Wizards do primarily damage makes perfect sense.
HP is (and always has been) an abstraction. It doesn't represent actual cuts, bruises, or burns, but more of a measure of "how close are you to making that creature never rise again." So you could use HP and damage to represent repeatedly slowing a creature until he stopped entirely--i.e., was at zero hit points. In other words, there is nothing to stop you from reflavoring your 4e fireball to be that you're actually turning the ground to mud and they are slowly sinking. It's all a matter of flavor. If you want him to feel like a BC rather than a Warmage, have at it. It just means the DM might have a few less conditions to keep track of.
BC is, in some ways, a different style of play. If the goal of a fight was to move a monster into a certain square, then BC-style tactics would be the way to adjudicate all powers. But the goal of a fight is to bring the monsters to 0 HP. So powers do damage. Whether we like it or not, it makes sense.
...at least it does to me.
EDIT: Alternatively (and this might be a better example, and easier to implement)--you could go with the "HP as fighting energy" explanation, and say that nothing except the killing blow actually connects. So maybe a "sleep" spell does HP damage because it costs the target some energy to shrug it off. In an rather simple game, that would make perfect sense (something at the complexity of say Inn-Fighting--there you could have a "sleep spell" ability that does HP damage because HP is the only thing that is tracked).
But this does break down as you add complexity, especially at the level of D&D. But one of the goals of 4e has been to reduce complexity so... This would be a way of reducing complexity but not necessarily dumbing-down the game (as long as the BC spells have other effects, or change the effectiveness of other abilities)--it becomes a game of tactical positioning rather than tactical spell selection (as I see it).