I'm not ignoring you at all. I think we're describing different experiences, and I'm failing to emphasize the pain point.
Interestingly, if it is EoC (and maybe abom after you destroy the soul vessels?) that has the extremely punishing dot zones, those serve a hugely different function in the fight. The outer ring degen zones in both those fights segment the arena and create big, telegraphed no-go zones. Presumably, to force you into closer proximity with the brusier bosses and prevent you from BVR sniping them into trivialized oblivion.
Cremorus' degen zones make the player engage with the main SFB mechanical gimmick of swapping barriers. The difference here is you MUST engage with the fire/necro fields in SFB. They're placed under you, swapped under you, and the boss is pummeling you with his spells the entire time. I'm unsure if there's no time between the fields switching or if the slow-fade vfx of the fight kinda disguise that "no degen" time. There's a telegraph that "a swap is coming soon", but the actual moment of the swap where you need to react and press the D key is either non exist or obscured to the point it may well be (to my aging gamer eyes anyhow).
Point being, if the player is forced to engage with a mechanic, it feels terrible to then take away their defensive agency in mitigating the results of said mechanic, even in part (the first couple ticks of damage after a field swap). It's much more appropriate IMO to use %hp removal in an "out of bounds" scenario.
Long asides not withstanding, I am 100% on board with different archetypes being threatened by different things. The point I'm attempting to make is this:
If Cremorus' fire/necrotic degen fields are in fact % current HP removal, I think that's an inappropriate use of that damage type and defensive mechanics should be applied to them.
If his fields are not the %hp degen, they hit like a train, and the character build mechanics that presently exist feel insufficient to deal with them. If this is specifically a SFB problem, giving players a clearer indicator that "It's time to swap barriers NOW" or allowing that swap to be performed mid-action like an instant cast spell would make it less frustrating.