This thread is about accessing a part of the Main storyline, which SE stated will be explained in FF14ARR, and not restricting a Major Plot point to Endgame content.
Current topic is...
"Keeping or not keeping the Difficulty of Binding Coil so only certain people can go into Coil"
This discussion has gone far off the original topic.
On topic of Endgame...
Most game I played keep the hardest content in endgame but they mostly only keep endgame storylines as origin stories or some kind of side story that will only have a small relation to the main story. Most common RPG endgame is the classic "100 floor Dungeon" which normally have hardly any story importance at all. Binding Coil and Crystal tower are made to serve as this games 100 floor dungeon but most people who run Binding Coil have zero interests in the storyline and only in it for the loot which is what Binding Coil is originally designed to serve as.
Giving access to a Major Plot point in the storyline shouldn't be a Major issue for people running endgame when this Story Mode doesn't harm endgame at all since Story Mode is not a Endgame content but a normal Solo Instance or a normal group dungeon for a Major Storyline. Not like the story Mode will give players Raid level gears when it will only give normal tier gears.
You still have your Binding Coil Raid mode with the Raid gears so there shouldn't be a problem since endgame is about getting the gears from raids.
Also we have the example of the Normal dungeon and upcoming Hard Mode dungeons. Hard Mode Dungeons will follow a different path from normal dungeons and the paths are also longer than normal dungeons with the addition of completely different bosses in Hard mode.
So Story Mode can follow this set up with Story Mode having completely different bosses and different path.
(I'll end my comments here since the current topic now is no long the original topic of the post)