An option for characters to start at level 50, pre-heavensward, for FREE

[Kaedan]

Because that's hundreds of hours of story to get through before you make it to endgame, and as has already been explained, some players don't care about story in an MMO, they care about endgame.
It's a large portion of their customerbase, who will feel forced to fork out extra money to play the game the way they want to play it.

Imagine if it was the other way around:
The base game and expansions unlock the world, jobs, and your endgame content, but if you want to play through the story, it's $5 per 'chapter'.

This is a double blow for those who fall somewhere in the middle, who don’t necessarily not care about the story, but want to reach endgame to play with their friends ASAP, and then maybe replay it for the story at a later date.
The story skip potions for them, is basically paying for the privilege of less content.

You completely missed the point.

New players who buy Shadowbringers will be getting $80 worth of expansions for free. That MORE than makes up for the cost of paying for an ARR story skip potion.

When 6.0 comes out, new players will get a whopping $120 worth of expansion free.

EDIT: And let me be clear. I have no issue with people who don't care about story having the option to do so. And they do. What I have an issue with is people wanting it for free, because that's just plain selfishness at that point.