iserith
Magic Wordsmith
Combat is inherently uncertain, so there is nothing for the DM to decide. The to hit roll is by rule, uncertain. Damage is by rule, uncertain. The result of the damage is by rule, uncertain. If the DM decides to change that, he is rendering the combat rules unreliable, as well as changing the rule.
Nothing is inherently uncertain in the game. Each and every instance of uncertainty is established by the DM. The rules are there to resolve uncertainty if and when the DM establishes it. Welcome to the era of DM empowerment.
As well, the DMG warns us there is a significant drawback to relying on dice rolls for almost everything - roleplaying can diminish if it's the dice, rather than the players' decisions and characterizations, that usually determine success.
Were I in your game, I could not rely on my attack working like the rules say, because at any time you could just decide that I certainly hit or missed. I could not rely on my longsword doing a d8 damage, because at any time you could decide that it certainly did a specific amount of damage. I also could not rely on my damage killing a creature like it should without me opting to knock it out, because at any time you could just decide that it certainly knocked the creature out. I could rely on no rules in your combat.
No, you can't rely on the rules. But you can rely on my reasonably consistent rulings in similar situations plus my descriptions. I do not abdicate my responsibility to determine uncertainty to the rules because they cannot account for the entirety of the fictional situation that is unfolding.
I think your position would certainly be more at home in D&D 3.Xe, D&D 4e, or perhaps a board game, but I don't think it best fits the D&D 5e paradigm. Of course, play that way if it's what you and yours like - nobody's stopping you.