Palace of the Dead
Floors 120+ disapprove your entire stance. Only a few mechanics one-shot yet most, if not all, can be interrupted in some manner or another. Wait, crowd control abilities being useful? Whoever would have thought!
What higher floors of PotD does reinforce is tanks requiring proper cooldown usage, healers to actually heal and for DPS to focus on specific targets lest they trigger a high damaging AoE. Compare that to Kugane Castle, where the boss mechanics are so minuscule, you can literally ignore everything. Yojimbo doesn't even do 50% of your HP if you ignore the coins he tosses around. Try that back when Stone Vigil normal was relevant and you'd likely have wiped the party. Why do tanks have to pull 10+ mobs to actually feel like tanks? If I go into Bardem's Mettle and do single pulls, the healer may as well have a red icon because they'll never cast a single heal. I've tested just how pitiful dungeons are. A while back, I played "ignore the tank" in Great Gubal. I didn't cast a single heal—not even Regen—until the mega pull. Had I not been DPSing, I would have been completely useless.
Shinryu Normal is perfectly fine. The issue is nothing scales from it. Every boss, Hashmal included, are easier from Rabanastre. Susano/Lakshmi EX are debatably harder and all of Omega normal rolls over. Has it ever occurred to you the reason people struggled with Shinryu and its EX counterpart is because nothing prepared people for the sudden spike? Many of us claim the former isn't difficult because he really isn't. He just demands you actually pay attention to mechanics, which is how all content outside maybe side story stuff, should be. If the majority of content allows people to ignore mechanics, they'll continue to do so until the game presents a sudden wall where everything they have done prior stops working. You want an example of scaling difficulty?
Ravana - > Sophia -> Nidhogg - > Sephirot - > Thordan
Done in that order you begin with a reasonable yet not overly challenging entry Primal, move onto one that prioritizes mechanical awareness to one that deals high damage into one combining the prior two and finishing off with a bit of everything. Despite being easier, Ravana still demands you respect his mechanics until you start to outgear him. Sophia may not be a big step above him, but she reinforces the need to pay attention. That is how you scale difficulty. It doesn't always come in a direct buff to outgoing damage. Now compare Susano and Lakshmi to Shinryu. The gap between them is enormous.
I also find it odd you seem to have all the answers while we simply "don't get it." Where are your Savage clears? How do you know so much about the differences yet have no experience to back up such an opinion?