Back then the only source of "lethality" was just one item (ghostblade). Not sure why they scrapped this smooth power curve for lethality stacking cancer.
Wrong there was The Brutalizer (probably the most oppressive stat stick in LoL history), Warrior Jungle item, and Black Cleaver.
Now let's take a look at other things you ignored. Lethality is gated by level. Old flat armor pen was not. There were also flat amounts of pen in runes AND masteries. A typical assassin would run flat armor pen reds. Let's do some math now shall we?
Flat armor pen reds: 12 flat (at level 1 mind you)
Piercing Thoughts mastery: 7% total armor pen
Compared to Sudden Impact which gives 10 Lethality which is further gated by level and only situationally
Let's move on shall we? You know what? Let's give you the benefit of a doubt and compare numbers at level 18 so you get the absolute highest value out of Lethality k? We'll use your 80 armor example
OLD:
Black Cleaver 30% shred brings them to 56
LW + Piercing thoughts brings them to 33
Youmuu + BC + flat runes brings them to -10
The max armor pen you could reach on a 80 armor target would be around 24+20
You see what I mean when I say you have absolutely zero clue wtf you're talking about?
Let's look at new:
BC 24% shred brings them to 61
Duskblade + Youmuus + Sudden Impact brings them to 15
You see now? This is a 3 item to 3 item comparison. This value is literally ONLY at level 18.
Again, you have absolutely zero idea wtf you're talking about. Did you even play the game at this point? Obviously not if you thought Youmuus was the only source of flat pen
EDIT: also forgot the old precision mastery which gave you another 10 flat pen at level 18 kek