Your probability balance concept as likely stems from the way our minds think as from anything else. Obviously the universe is in constant change and so nothing at all seems truly permanent. But what if any direct connection is there between that observation and the idea that human development operates of some sort of seesaw between things that are opposites?
Why does good invoke evil — if indeed it does? I think the apparent balance of opposites is in part illusion. If you are starving, a scrap of dry bread is wonderful — but in a fancy restaurant you might turn your nose up at it. Motivation constantly pulls us here and there but, as we are finite beings, we are forced to leave one situation in order to experience another. So we tend to be forever balancing things in a cognitive manner — calling the same scrap of bread wonderful or no good according to circumstance. So even in a harmonious and peaceful world we might still find something to complain about. But that is different from saying that good and evil are inherently in balance and therefore that harmonious and peaceful world cannot be achieved.