Genuinely difficult content (Extreme, Savage, and Ultimate) is there for players who want it. The MSQ should not be designed to kill you; this game's focus is on it's story, whether or not you like a given line, and all content is set to "Easy Mode" by default. Besides which, you don't have a full toolbox below the level cap - and because of that ever growing toolbox, even simple 6.X bosses can be more complex than high difficulty 2.X ones.

Difficult story bosses can be incredibly rewarding (hello pre-nerf Shinryu!) but it's not the wisest course for drawing in and retaining non-hardcore players, and as an MMO there needs to be as wide an audience as possible.