Why encounters with a difficulty slope are bad

[Ibi]

TLDR;

Please stop your old Square Enix Pattern that you discovered in FFX for the first time (Seymoure anyone?) and try to design more interesting encounters instead of just piling on more of already existing things to simulate non existing difficulty.

Good Design:

T1, T4
Garuda Ex, Shiva Ex, Levi Ex, Mog Ex, Odin
T7, T8

T7 and T8 both "just pil[e] on more of already existing things to simulate non existing difficulty":

T7 adds Cursed Shriek, the DoT/exploding floor mechanic, Venomous Tail, and Petrifaction as the fight goes on.
T8 adds land mines, Brain Jack, Ballistic Missile, and Allagan Field as the fight goes on.

Given your own criteria, I'm not clear on what makes you think those are good fights, versus T6 or Final Coil.