Your theory is wrong - I think the metric you made up to analyze the value of redux shows where your thinking is going astray.

Imagine a fight vs a thirsting rager. Let's say he does 0-800 damage per round, so you attempt to flee when your hp is below 800 so as to reduce the chance that he has a big round and kills you. He doesn't flee ever. Now, many of the fights that you win will be ones where you are above 800 hit points - does that necessarily mean there isn't a need for dam redux? No - in fact, dam redux makes the game better, because the hp totals become lower (because the amount of combat that can take place before a nonzero chance of death kicks in is longer) giving the illusion of closer and more thrilling fights, and with longer combat comes more tactical options.

Ever get one rounded by a villager, and think "man, this is so dumb. they are so overpowered!"? The villager knows that it's an incredible fluke of luck to hit 3 DB's in the same combat round. Now, imagine that everyone had 50% dam redux - that opening salvo would only take you to 50%. The effect of luck on the game is reduced. Imagine a game where everyone has 400 hp - there would be quite a bit less stragety, people would be afraid to fight because every encounter is potentially death - surprise opening attacks become incredibly powerful, and the entire combat changes. It would be thief standing in one room spamming backstab, afraid to leave the room knowing that other player is spamming bash in the other room, and knowing that whoever gets the first hit wins.