How Flat Is Too Flat?

After all the problems we saw with the aggressive scaling of bonuses in 4e, I'm very much in favor of the flat math system they're using for D&D Next. But how flat is too flat? With the new proficiency system that I discussed the other day, characters receive a flat proficiency bonus to attack rolls using weapons with which they are proficient. This means that if a fighter and a wizard are both proficient with a given weapon, they will have the same attack bonus unless one character has a higher Strength (for melee) or Dexterity (for ranged) bonus. Obviously, the fighter is much more likely to have the higher bonus, but it seems to me that the fighter needs to be inherently better than the wizard at fighting; his superiority should not be solely a function of a higher ability score bonus. Maybe we need to resurrect THAC0 after all...