Can you actually prove that they can penetrate metal, though? Because that's what we're discussing here. Also, it's interesting that you cite modern studies while neglecting recreations of medieval fencing manuals. You keep trying to twist and reframe things to suit you without providing any sort of proof that a sword (specifically an arming sword) can penetrate a steel harness by an alleged NINE inches.

So. Let's read a bit about the Type XV, shall we? I'm going to assume, for the sake of argument, that MyArmoury.com is an adequate source for your very particular tastes. It came about in the late 13th century. There was no full plate harnesses around at that point. It lasted throughout the 14th century which was marked by the development of transitional armour, but still no full plate harnesses.

During the 15th century we start seeing plate armour, but guess what? We're also getting weapons that are far better designed at dealing with armoured opponents, such as the pollaxe and the warhammer. The days of the mounted knight fighting with sword, shield and lance are practically gone. Knights of the day are apparently so confident in their armour that they often choose to eschew the shield in favour of two-handed weapons.

The Type XV sword is a thrusting weapon, but that does not by any means imply that they can thrust through a late 15th century harness, and certainly not your alleged nine inches. There is a reason for why contemporary fencing manuals teach you all these grappling techniques rather than telling you to simply stab the shit out of your opponent.

If you've got a sword and you're fighting an armoured opponent go for the joints. Go for the gaps in his armour, his armpits, his eyes, his chin. Grab your sword by the blade and bash him in the head with your crossguard or your pommel, but trying to stab through plate is just stupid.

I'm actually going to step back from my initial statement that you will never ever penetrate plate armour with a sword, simply because there may be freak occurrences, but using it as your primary method of killing someone? No.