That's assumes perfect information and perfect play. As new strats and counters are discovered the relative power of characters shifts over time.
Balance doesn't have to do with changes in perceptions of character strength. A game is usually said to be balanced if there is a large selection of characters that are considered viable for tournament play.
I think this distinction is important. Balance is about the diversity of choices players can make.
In nature, there are balancing mechanisms, but there is no balance. If one organism learns how to overwhelm another, it often spells the extinction of the weaker organism.
In games, however, if one character is unviable, then the game is said to be unbalanced, which naturally leads to discontent with the portion of the player base that likes that character. Balance is a battle against the natural entropy that generates tier lists or food chains, because it is a human preference, not a trend or a law of nature.
Balance doesn't have to do with changes in perceptions of character strength. A game is usually said to be balanced if there is a large selection of characters that are considered viable for tournament play.