Proper Heat Treat and Temper does not mean as hard as possible.
It means the proper final hardness for the Steel and the intended use of the knife.
Harder is not better for most knife applications.
In fact on a lot of steels-- harder--means chipping issues.
As you increase hardness you increase Strength--Lack of Flex--Brittleness.
You decrease Toughness at the same time.
Proper heat Treat and Temper is that balance of those properties.
Most of the Stels we use in making knives have an optimum range of hardness for use..
It does vary with each steel and can be adjusted for tasks but most steels are in the 56 to 60 range of hardness.
That is a Huge range.
56 is actually considered kind of Soft and 60 is considered very hard but there are a lot of steels that can be used in that entire Range.
As You know--hardness is only one issue in edge holding and ease of sharpening.
Crossectional Geometry and Abrasion Resistance of the individual steel will be big factors on the performance and the ease of maintaining the edge too.