In-Game Parser.

[Dwill]

I'm refuting that a "basic job rotation" exists in a meaningful sense at endgame, which when you look objectively at the rules of each fight is absolutely true, there isn't one because the mechanics of the fight prevent that. Are players that perform poorly at their class' mechanics (loosing buffs, not keeping dots up, poor management of OGCD abilities) going to have that performance mirrored overall? Yes, and I'm not refuting that. But how does that logically transform a heuristic flowchart into a rotation?

We can argue semantics if you want but for every DPS class out there, there exists a rotation. Said rotation though can barely ever be followed as you mentioned due to fight mechanics which means that now once your opener's done, the priority system kicks in. What people are asking is for people to know what said rotation is so that they can better adapt in actual, non-dummy DPS scenarios and then can properly manage their class's priority system. If people can't be bothered to learn that, then they probably can't be bothered to learn a fight's mechanic which just creates a bad domino effect.

More importantly, isn't "perfect adaptation to every possible scenario" the desired endstate behind adding a parser? To make those anchors you carry around able to carry their own weight?

The endstate behind the parser is being able to better oneself as a player, not to attain perfection. As such, this answer your second question. People should carry their own weight, not drag down a group and a parser helps tremendously in that department.