Okay, let's back up.

FFXIV is marketed as a story-based game. Comes with the franchise. This is hardly disputed.

Therefore, most development time is put into the MSQ and the instances that accompany it.

What exactly is making it a side-story optional going to accomplish? Essentially there is now less game to play. Less people have instances unlocked to play with, queues get long, things die, people leave. How is that helping the community? How does that keep subscriptions? How does that keep content alive, when the devs spent time and resources on it?

It doesn't, it leads to a very unhealthy game. The economic repercussions are pretty serious.

Now, if you want to talk about if introducing a leveling potion would be good for the community? You can claim people can learn how to play their job from the tooltips. But is it true in practice? Do you remember people complaining about tanks that leveled up through fategrinding and killing mobs and had no idea what their role was in an instance, and didn't know how to generate hate? What about the DRKs/ASTs right after HW dropped queueing into leveling roulettes with no idea how to play their role? Was that good? Was that fun? Was that healthy? Yeah, the tooltip argument doesn't work in practice, and that's all that matters.