I think it is revealing to consider the history of all this and how HOI got to where it is today.
Once upon a time, hardness in HOI was all or nothing. Divisions were either hard or soft and I built mechanised divisions because "why build tanks" and so hardness as a percentage was born.
Then we had the supremacy of attack. Barring real bad terrain the optimum tactical choice at all times was to attack with all of your divisions. The operational objective, at all times, was simply to have more divisions engaged in fighting (well, attacking) than the enemy and so breakthrough was born.
There weren't any stacking limits so the easiest way to exploit the supremacy of attack was to rush around with mega-stacks killing enemy units and leave your lines wide open and so combat width was born.
Also, it turned out that because the enemy army always had a lot more soft stuff than hard stuff the optimum strategy was to maximise soft attack and forget about hard attack. The enemy tanks might be a bit of a grind when all those armoured divisions had some softness you could kill. No need for any anti-tank capability at all and so armour and piercing was born.
All those changes made by Paradox have achieved their objectives and the game is far closer to an ideal model than it was prior to HOI4. We have game where defence works, anti-tank capability is necessary, tank divisions are important, creating powerful breakthrough units is a thing.
We are now onto the next thing which is, by comparison, relatively minor. Land combat isn't perfect but the remaining flaws are tiny compared to what it once was.
But, yes, it would be really nice if we could have graded armour penetration for both realism and game playing reasons.