If the MSQ was not required to do dungeons and raids, and was not required to access the expansion, would you quit the game? If you didn't play the game, and someone told you that the story content was there, but optional, would you decide not to play the game because of that?

I highly doubt the honest answer to either of these questions is "yes."

And yet, the opposite is not true. There are many people who will quit the game out of frustration stemming from the MSQ grind. There are many people who will dismiss the game after being told they will need to do 30+ hours of mostly fetch quests before they get to start the 20+ hours of mostly fetch quests in the expansion before they get to do end game content.

What is the benefit of forcing everyone to do the main story quests? It certainly doesn't make more people understand the world and the story. I skip everything and I honestly have no idea who any of the characters are outside of their appearance. I don't know why we're going to Ishgard. I don't care. Forcing me to do tedious fetch quests will not make me understand or care about the story that I don't want to experience in the first place.

What's the point? How will it hurt you, if you enjoy the story, for it to be made optional?