Ok, guess fisking is the proper way to respond to this.
So, just to be clear here. You are equating changing a single die roll result to auto killing a PC? To automatically failing every single check that a player could make? I just want to be absolutely clear that this is the comparison you are making.
Nope. This is false. There are numerous reroll mechanics where the better of two results are chosen. There are also numerous mechanics that simply change the target number needed - Shield being the clearest example of this. So, no, you do not always replace the first roll with the second.
Just like fudging.
Just like fudging - although, to be fair, the when and how are largely left up to the DM.
Except when there are numerous mechanics that change the first rolled outcome flat out. Which you seem to be continuously ignoring for some reason that I cannot quite understand.
But, yes, I do agree that I'm moving to the conclusion. I'm not sure where you are getting the notion of "justification" from though. It's not about justifying anything. I don't have to justify anything. It's right there in the rules. As far as I'm concerned, there just isn't really any difference. In both cases, the results are altered after the fact. And, realistically, the player side mechanics are far, far more used than DM fudging. I seriously doubt that any DM fudged every single encounter. But, there will be reroll style mechanics used every single encounter. Making it player facing simply made it a LOT more common in the game.
And, that's fine because the player side mechanics are generally quite fun. So, great. It's a better idea. Moving fudging from behind to the DM's screen to the player's hands has certainly resulted in a better game, IMO. I'll not argue that it's worse.