It's about time to add Coil story mode, isn't it ?

[Teuciont]

It's so weird that Coil story seems to be the "true ending" of the game, thus more than half of the playerbase cannot enjoy it.

next endgame raid should be completely optional story that has nothing to do with the main story at all.

They went out of their way to disassociate coil with the main story completely. It is in no way the "true ending of the game." You might be able to make that argument for Legacy players, but even then the focus of the ARR story is totally gone from Bahamut. He's mentioned in passing and while yes, you are spending a great deal of time hearing complaints about what the Calamity did to the land, neither the Calamity or Bahamut are of any focus past the first fifteen or so levels. If they are mentioned, it is only briefly. I would argue they designed it this way to prevent people saying that it was essential to beat to understand the story. Coil's the super hard optional dungeon in your typical JRPG. You could never step foot into coil and still fully understand and comprehend everything in the MSQ.

Edit:
I'd also like to add that if you completely erase any important connection to the world itself then you severely detract from any excitement that might be associated with that content. Alexander's already questionable in terms of relevancy to anything we've seen so far happening in Eorzea. While Bahamut was technically an important part of the 1.0 story, it certainly isn't anything your average new person to ARR has to worry about.