A system where X>A>B>C>D is INHUMAN and unrealistic.
No, Oda didn't introduced Haki because he lacked something. Haki was here since the beginning. Chapter 1: Shanks traumatizing the seabeast with conqueror.
The system of One Piece is great in the sence that it is NOT a power system.
Devil fruits are not here to create superiorities between the characters, they are here to EVEN the plainfield. In fact Haki is here to do the same with non devil fruit users.
The system of One Piece is great because it's a deeply equal system. This is a system where even the most ungifted individuals can match with people with tromendus power.
The moral of One Piece is that if you take your power for granted or if you don't care enough to pay attention, you will be put down like any human in real life. That is why Luffy can still be put down by random devil fruit users or random people like Atlas.
The only exceptions to this system are the two most powerful individuals that are shown as being strong despite themself: Big Mom and Kaido. They are written to have inhuman capacities but highly human minds. So their strenght overflow their own body and their own mind despite their will. While BigMom can't help but be a impenetrable being but an higly instable one, Kaido can't help but being the strongest beast of all. That's why it was important for both of them to be put down at the same time. All of that is why you can see Queen impact Big Mom on the floor: the body of Big Mom is inhuman so she is unhurt, but her mind is instable so she regained her memories.
Any other character would be put down by Perona's amazing power, because almost all the characters of One Piece are human.
So here is my conclusion: One Piece is the opposite of that power system you guys are seeking. That system just can't be done in the world of One Piece, it's contrary to it's values.
Devil fruit powers is an amazing system, with 3 well balanced categories and the haki system is the perfect respounce to that system. All of that ends up in a perfect equilibrium and keep the characters human.
So their is nothing wrong with that system. It's in fact much better than what we can see in most manga.