Please keep in mind, that the 40k System is a different system than what we are using here. You only penetrated the armour, if your AP value was the same or lower than the Armour itself (so a 3+AS needed a AP(3) or lower so that the armour stops working).yes, all arguments used when 3rd ed. 40k hit. They where all proven to be disliked by the community, and as 8th ed hit, reversing it, has been one of the most welcomed and natural changes. But, by all means, feel free to gnore history and give it a whirl.
More design space? I disagree, you could do the same thing with the exsisting rules or a special rule for the few units who needed it.
It didn't work that well tbh.
However, we still have the armour MODIFICATION, so you still modify the armour with your AP value and not ignore either the Armour or the AP, just because one exceeds the other.
Regarding design space, sure, you could always create special rules, which can imitate those design splits. However, do I really want to have a boatload of special rules just to get to the same result which a Characteristic split would do on its own?
I would rather take the Split so I can simply look it up on the statline and don't need to dig through the BRB to find that annoying special rule, when I'm not exactly sure, on what it does.