>>but it increases the edge cross section immediately where as very tiny bits of a stronger but less tough edge steel (or even equal strength, but less tough) will not increase the edge cross section. <<

probably not obvious what I meant. Extreme toughness allows the edge to roll even in a hard steel and even though it starts small at suitable hardness, it holds on and propagates. It also seems to go deeper than chipping does, or at least as deep. 

when an edge is softer (like too soft, not 1820 soft, but like not enough carbon 2022 soft), in my planing test, the softer the iron, the deeper the damage. It's problematic on plane irons, too - the deflected edge won't enter wood. a few small chips and you can keep planing (it always lowers productivity if the chips split the shaving - by a lot). When I ran into maple, just a few small dents with deflection of a few thousandths each prevent a plane from continuing to cut even in an ideal test. 

Nobody probably cares much about this stuff because the actual use of hand tools is far less than the talk about them and purchasing of them. 

They would've affected income 200 years ago.