katanas weren't great because of the folded metal - everyone was folding metal. katanas were great because of the differential temper.
That's nothing special either. Even ancient romans used that technique.
this is why it's more armor piercing. i personally have cut through medieval armor with my katana. just because YOU can't, doesn't mean nobody can.
What kind of armor? Hide? Leather? Cloth? Paper?
You can't
cut through chainmail ... you'll dent some links at best. end of story. You can pierce it, but that's something completely different.
all katanas had to stab by definition - that's why there's a hand guard.
By definition a curved sword is for slashing. I don't know why I even have to point out something that obvious. Sure, you can stab with it and it's completely sufficient against unarmored or lightly armored people. But once you have to penetrate something more challenging you are better off with something straight and pointy.
when you stab downward to finish an opponent, the handguard protects your hand from your blade. as far as piercing techniques go, japan's greatest samurai (soujiro) used a 3 attack piercing technique which was considered the strongest technique by far. soujiro was ranked 1 of all time. saito hajime was ranked 3 of all time, and also used a piercing technique. so the katana was used extensively as a piercing weapon.
no big surprise. europeans discovered that stabbing from far away is sufficient for killing unarmored opponents, which is why the development of sword forms didn't stop with the longsword and lighter, faster weapons soon dominated the duel grounds like the rapier.
japan reached 'perfection' and stuck with it.
To be fair, japan probably couldn't have made the switch to rapier-like weapons even if they wanted to, because their local iron svcked and was brittle as hell. smithing a long, thin, strong and flexible blade was simply impossible for japanese swordsmiths without importing better iron ore.