That's a fair point! Something else that isn't being mentioned: all expansion storylines are not created equal, and that is subjective.
Everybody has a storyline they love, a storyline they hate and ones that are in the middle. For some people, their favorite storyline might be ARR and then they don't enjoy it again until Shadowbringers. My point is that there is no safe way to skip expansion storylines when they are all connected. The only exception would be people that don't care about the story.
How about this idea? If somebody is not interested in the story, they can start in the current patch (and still have to pay for the prior expansions). Except that they would also be required to do short training dungeons with Duty Support. One every 5 levels until they reach level 80. Each job would have its own unique set of training dungeons. If they fail the mechanics, they have to try again.
Frankly, I wish we had that option for everybody!