If raids were just simple re-action and get out of the fire mechanics that you could one shot, this game would be boring as hell.

I like that they extend skill in MMO's to more than just how fast can you move out of the red shit, not to mention those with latency would get crushed if that's all it came down too.
Yes, you have to learn a boss encounter, heaven forbid you have to pay attention and learn mechanics.

Just because you know the encounter, doesn't mean you can down it, I am sure many people have watched a million T9 videos but can't finish it off. There are people who can't compensate for a messed up RNG and go in for a double fire in even when they haven't had an ice and wonder why they died because they didn't look at their debuff bar and compensate, are you able to mark dive bombs OP? Sure they are set patterns but you have to be able to notice the current pattern as it changes and place marks accordingly, can you do that?

T9 is very easy now with echo and gear and it is definitely relaxing, but when you had to handle mechanics and deal with having lower gear, and still keep up the DPS and not get face rolled then it was harder.

T13 is great, its still very much scripted, but the megaflares require attention and reactions depending on what part of the mechanic you get.

TL;DR

I don't like what you are proposing, I prefer to have mechanics that can screw you over and you need to learn because if I could clear a fight just by reactions, then all it would mean is I need to just dodge and that....is boring.