RadmanSure, but then valve placed medic as the 1 solution to the problem of overly defensive classes. Which means any subsequent unlock that can potentially buff the heavy or engie (or any sort of defensive play) throws the game out of whack. And anything that makes medic stronger will add power to an already objectively overpowered class.
They painted themselves into a corner in terms of design space. And the diversity of high level play is suffering for it.
Overwatch is already fixing this problem: while supports are still necessary, almost every class has their "uber"; a big ability that helps them push and break defenses.
Medic is overpowered in the same way queen is overpowered in chess, he is the most important piece of any team constellation and it is necessary to keep him alive. However, this is what makes TF2 interesting, the medic dictates the flow of the game and you have to play around that class. As I said, without the medic TF2 would be just another arena shooter.
Diversity is not necessarily a good thing in competitive FPS. Quake and Counter-Strike are examples of games that are not diverse (i.e. repetitive), but are good competitive shooters anyway. Overwatch may have an alternative solution, but that solution completely changes the game; it is simply not the same game anymore if you applied it to TF2.