I fully support SE implementing more random damage spikes. Hell, just making fights more random. But SE scripts their fights down to the tiniest details you'd think they we're making movies instead of challenging encounters.
It's just a big round of "Simon Says". It's how they design encounters. It lets them make sure they're "Exactly this difficult". But if it's always exactly that difficult, then it's always that predictable. And predictability is boring. "Simon says its time to heal! Simon says move here. Simon says DPS, then DPS while moving! Simon says move again, then Heal."
SE is so adamant about forcing you to stick to the script that they just guarantee you get killed if you mess up more than once. They do this in the form of Vulnerability stacks. Get hit once? You'll live, but heres a Vulnerability stack. So if you get hit again, you're likely gonna get 1-Hit KO. This does two things: 1) Ensures you can't cheese through mechanics by just healing through them. 2) Forces SE to make the damage/mechanic avoidable, making healing less relevant. I can't heal if people don't get hurt.
Some damage is unavoidable, but it's so pitiful that everything solves itself in one GCD. So taking "scripted damage" is just a nuisance, not a life threatening situation; and thus, trivial and boring.
At one point, SE used to design parts of the encounter specifically around healers. Like Ifrit EX tethers or Leviathan EX healing debuff. I don't think there was a single mechanic in any of the Stormblood fights that inhibited, changed, or impacted healers in a meaningful way. Tanks get Tank Swaps and Tank busters. Healers get... to just move around and dodge things like DPS do? Where are our mechanics?