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

[Seraphor]

Why should ARR skip become free when new players are getting previous expansions for free with each new expansion?

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.