I do understand that appeal, I agreed that bosses are fun and the mechanics are great, and I do love the challenge of figuring out bosses, formulating strategies, etc... we want phases and some predictability of course BUT you are taking a total extreme here, I am not, and I hope nobody is advocating for completely random fights!
The OP and I both believe that the mechanics are too unforgiving, and there needs to be a better balance between mechanics, DPS checks and "some" (not complete) randomness.
- There should be some things that I can expect coming, and plan to dodge and memorize for, such as phases, mechanics and coordinated telegraphed attacks, but if it hits me it should not wipe the party.
- There should also be elements that I need to react to, and therefore are reliant on reflexes, not memorization
The benefit from this balance is that items are more relevant, if someone else fails to dodge the mechanic the party doesn't wipe and with good enough gear you can overcome that setback. If you fail to dodge a random attack through a small mistake you should not wipe either.
All FFXI players come from a game where there is a healthy balance between mechanics and reactionary situations, all these challenges can be surmounted with good enough gear, but only months after the content is released. This way the hardcore get a chance to beat it at the start and when they get better gear, they can help their guildmates through it later. This gives me an incentive to obtain that gear, which is also absent from the game due to the heavy reliance on insta-kill mechanics. In Second Coil, insta-kill mechanics are fine since this is the highest tier end-game, but why are storyline fights like this too? Like the OP says it leads to more frustration than there should be and some people giving up.
If SQEnix gave me an item, that enabled Tempered Will (knockback resist, probably specific to Earth damage at a certain level for balance purposes) on a 30 second timer, imagine the instant appeal! It should only be obtainable through far harder content, but still I would aim for that immediately, why? Because I have tons of friends in my LS that still have a hard time clearing Titan EX. But right now there is no such item, Titan EX you win or lose based on mechanics, if just a few people make one mistake, then no item and nobody can salvage the fight.