I think what Farago means is that when you run your ki bar runs out it keeps going down, not your power level.
But that's a good idea, so maybe your power level AND life force could go down when you reach negative ki?
That would make the game more fair, so high power leveled players could be beaten in a continuous onslaught.
After all, nobody in the anime ever managed to fight for ages and not get tired, even against reasonably weak opponents.