Would a classic full time armageddon tiebreaker be balanced? I think that classic chess and rapid chess are two quite different kinds of sport. The classic chess title was undecided this year 2018.

Why not use the armageddon rule, that black wins if tied in exchange for a white time advantage, in a classic format? Say for example normal classic time rules for white, but in total only 100 minutes with no increment per move and no additional time after move 40 for black. Could something like that be fair or is it too easy for black to force a drawing position during the first 40 moves? Is armageddon all about blunders under time pressure?
That wouldn't make any sense. In a long game between two 2800+ players, a draw is always very likely. Carlsen and Caruana understand exactly how to steer a game to a draw.

Whoever gets Black would have a winning advantage before the game even started.