But we're talking about vanilla, not just a single DK ability. In Vanilla most specs were useless and most classes were relegated a single role to play competitively. You're also ignoring all the huge changes to itemization and other systems throughout the expansions and patches so that the Frost DK in your argument only needs a single ability change to attain that "Frost DKs were fine in all their abilities"
Are you naive enough to believe that simply changing the damage of class abilities will be enough to attain class balance for all the classes in Vanilla WoW?