First off, when I said Universal Win Condition, I was referring to the removal of all your opponents stocks before your stocks are all removed. We can agree that raising your opponent's percentage is typically a reasonable measure of success throughout the match, and generally makes it a bit easier to remove the stock. Of course, there are factors such as stale moves, etc, but in the flow of gameplay, as you TRY to win, the stock and percentage raise. Under your system, I could merely camp, and avoid death, while being in the air LESS than you, WITHOUT ATTEMPTING to damage you whatsoever, and still win. BTW, edge time does not count as air time. You must use all the same tactics which you would employ to defeat the opponent, to time them out, merely on a more defensive stance.
I can make up arbitrary rules like, "When the timer runs out, the higher tiered character wins the match." Nobody needs deep analyasis to realize this is a bad rule. I could say, "the player who used the least projectiles . . ." as projectiles are often campy and defensive. However, you can still see this is instantly a bad rule.
Additionally, I have shown you that there are some characters like Olimar, Snake, and ICs, which often force you into the air, while having very grounded games. These are characters likely to time you out. Why should they receive such an arbitrary bonus? King Dedede and Diddy Kong are more good examples.
Also, the reason that the timer is implemented into the game is because it causes an eventual victor (so tournaments do not run forever). Why should the eventual victor not be the one who was nearer to defeating their opponent? Air time is a HORRIBLE basis for determining anything? MK is pretty broken, but why should he be penalized for simply flying? Why should characters like Marth, who rely on jumping, inherently suffer?
The rule is simply and obviously poor.