Hear me out for a second here... So much of the stuff you come out with is based on opinions right? You yourself have openly stated that you didn't raid coils when they were current and whilst I'm not sure if if you did Gordias Savage, my guess is that you also didn't. That's fine, that's absolutely not a problem. I'm not trying to bash you with that, I'm trying to understand your perspective of things. Am I correct with this assumption?
The problems arise when you swing your weight around in discussions with these in content that you never actually took part in at it's peak. It's like the Warhammer comment where you come out with a random comment that's demonstrably incorrect with a simple google search. It undermines your argument and when your post has a bunch of errors like this, you end up with people like me coming along and bonking it with the 'correction' sledgehammer.
Case in point, the reason why I asked if you thought high end healers were chain casting cures in Coil is because I don't think you understand how top healers optimised it back then, and honestly I don't blame you, it was a weird time, up there with day 1 dungeon strats tbh.
Let's go with a Mr Happy video:
https://youtu.be/H75f6SgM6Sg?si=-M9i1ONjZ4eDtF3K&t=67
I'm pretty sure his healer for that was called Miunih Evans, top notch healer. If you watch in the bottom right of the screen at 1:07-1:10. Watch Miunih's cure animation closely. Do you see what he's doing there? He does this a lot through the fight and it's unfortunate it's not his PoV but if you carefully track him throughout the video, you'll see the same thing repeatedly.
Basically, he was pre casting Cures even when they weren't needed. If it got to 50% and it didn't look like the cure was going to be needed, he canceled it and restarted the cast.
Why? Because MP was the great bottleneck early on in a coil tier. Incoming damage wasn't actually all that high, however it was so much erratic than what we see now, and those spikes could flatten a tank in the time it took your GCD to cast because as you point out, we didn't have the instants and oGCDs to react in the same way we do now. Letting those casts finish would cause problems with MP and enmity, so the MLG play was to start then casting but interrupt them halfway through if they weren't going to be needed.
Remember, crits on tanks were a thing, most bosses (Maybe all?) had a mini tank buster that wasn't telegraphed and thanks to the lack of long cast times, many bosses did considerable auto attacks right the way through mechanics and tank busters. Twintainia was probably the pinnacle of this. Death Sentence in isolation wasn't the worst tank buster we've seen by far. It was the fact that it would be followed instantly by both a Plummet and an Auto Attack (which could crit). Because this is a sequence of hits, now factor in Parry/Block/Dodge odds and hopefully you start getting the picture.
The TLDR here: Healers weren't playing it super safe because healing requirements were so much higher in Coil because they honestly weren't, they were playing it safe because damage was spiky and inconsistent, resources were extremely tight, enmity was problematic. Some healers figured out that they could 'animation cancel' a cure to save MP if it wasn't going to be needed, some healers would just stand and wait.
Before you hype up coil as having these huge healing requirements, go watch a T1 prog pov, it's actually pretty grim viewing in hindsight. It's not until Caduceus suddenly strings together some crits and the tanks fail to parry/block them that things get spicy and faces get melted. Outside of those windows, it's really not the golden land even if my rose tinted glasses would like to think it is.
Compare the hood swings on Apache at 0:37 seconds vs 1:00
https://youtu.be/VZDTK1Q10GE?si=fi_6CNuLl9rYQ0Qc&t=34
https://youtu.be/VZDTK1Q10GE?si=fi_6CNuLl9rYQ0Qc&t=55
One sequence has the tank down to 30%ish HP, the other barely makes a dent taking the tank down to maybe 80%? Then you get a whole lot of not much until 1:17 when another connects and doesn't get blocked etc.
Hopefully that gives you some more insight.