About the flat vs % based damage reduction:
As far as I recall there IS actually a minimum damage threshold (5) for damage, so it can never be reduced to 0. So you could actually kill the guy in plate armor with a table spoon, given that you are nimble enough to avoid his hits (because a single one will devastate you). It's actually quite easy to grind some of the wildlife down with your basic sword because they have very easy patterns (if you don't run out of patience beforehand). Humans with combat training on the other hand have some erratic and very fast moves, so it's almost impossible to totally avoid getting hit.
What critics or the armor/damage system don't realize is that it is essential for the Gothic feel. Gear and skill upgrades are very noticeable and give you a better sense of progression. With your shiny new guild armor those goblins suddenly can't overwhelm you anymore because their makeshift weapons can't pierce it. After putting some points in strenght and getting that new axe you are finally dealing REAL damage to that troll, time to try him again.
This won't happen when upgrading from Shade armor to Gardist armor would net you +10% damage reduction. The difference is there but barely noticeable during a single fight sequence.
And the feeling of progression is why the Gothic games were so great. Everyone loves the early and mid game, because you explore and get noticeably more capable with every milestone, giving you new exploration options in turn.
Cleaving up a wolf pack that destroyed you beforehand with your new sword and armor feels amazing.
Of course this system has problems as well. There is an upper ceiling from where the scaling doesn't work anymore. Most of the content becomes trivial and you become overpowered. Nobody can hurt you or (in case of Gothic) you destroy everything in one hit. The Gothic games suffer from this as well, especially in the end game. When you focus your build, nothing can stop you later in the game. It's becoming boring and exploration options narrowing down are not helping either. That's part of the reason why the last chapters are not liked well.
A percentage based system would provide with an overall more balanced gameplay during every step of the game. But Gothic is not balanced and the lack of balance is (part of) what makes it so great.
A good example for a perfectly balanced game with no OP shit and decent percentages would be Pillars of Eternity. Who doesn't love increasing their damage by 1%?
On the point of flat reduction being unrealistic, percentage based is not better. Your plate mail providing 50% reduction against spoon attacks and nuclear warheads makes no sense either. It doesn't really have to, though, because it's a game system and if the gameplay is for the better with it, don't hold back. Just don't implement it in the Gothic remake and think players will feel the same as back then.