> Unlikely!? Doing damage with a knife is tough? Yeah, 4 inches of steel in your chest is no problem, lol, walk it off.
It would hurt quite a bit yea.
Any knife fighter will also tell you that hitting a skilled opponent there is bloody hard :)
Bearing in mind how an unskilled person will almost certainly hold the knife (pointed straight at you) them hitting you in the chest requires a serious fuck up on your part :) There are 2 defence options (lets assume a right handed attacker):
1. Side step the knife on their off-body side (so they have no way to swing their arm at you without reversing grip - which will take too long). Take their hand in a standard grip (I'll dig out some pictures later) with your right hand and pull the knife/arm "forward" (i.e. into the place you were just standing). Reverse left elbow into their face to stun them, pull left hand forward to pressure point a knife released. This should take 2 or 3 seconds at most. They will either be on their knees behind you (kick the knife away btw) in which case you can push them sideways (not forward - that will put them potentially near the knife) and put them into a lock. If they are still stood you could do all manner of things.
2. If your better and prefer a more violent approach you can step onto their on-side inside their arm. Take the wrist in a lock with your left hand and push outwards (let them bend the arm up if they try too, it gives little extra leverage). Hit them really hard in one of the open spots on their body with your right. Preffered hit is the heel of the hand into the kneck just under the jaw. Disadvantage is a) it might break their neck and b) often they will put their head down. Side of the neck is probably your next best target. Again this should take seconds.
> There may be a time in my life where I get robbed with a knife wielder. I'll assess the situation, and I might even take action. But it'll still be the stupidest decision of my life, and I might have to pay for it.
Are you trained in unarmed defence? Have you practiced disarming knife holders? Are you able to react instantly and violently?
If the answer to any that is less than a Yes then you, clearly, shouldn't be doing this. And I've never said otherwise.
The Israeli in the OP clearly was a Yes for all of those. I am confident I am enough of a yes to take on unskilled combatants like the one sketched out in the OP.
And I have been held at knife point. At that time someone else took action but I was prepared too if needed.