If the story content is optional, then people wouldn't play it for more than a few hours. They would play it just long enough to get to the logical end of killing the hardest monster... forget about the dungeons and raids.
Part of the game's design is that the entire point of doing the dungeons in the first place is that something in the storyline requires it. If you skipped everything, that is your loss. The storyline content has to be weaved in with gatekeeping the content to keep people from just grinding in one place.
If you notice, the average dungeon only needs to be run 3 times to level out of it so you can play the next one. Once you unlock everything, you can play any other job/class and not have to unlock them again. IN OTHER GAMES YOU HAVE TO CREATE A NEW CHARACTER AND UNLOCK ALL THAT STUFF AGAIN. So instead of complaining that you have to unlock this stuff at all, look at how the armory system removes the need to do all the main storyline 12 times.
But again, in some other games, the ability to actually play all the dungeons is available at level 1, but people just grind on the one dungeon that is the most experience efficient and ignore everything else. And for what? Get to the maximum level and then ... grind a new character?
In other games player typically won't do content that isn't required. Such is the case with Crystal Tower and The Binding Coil of Bahamut, and the Extreme Primals. In FFXIV this content is "hard" content. The fact that DPS have such long queues for any dungeon is indicative how much content players are NOT participating in. The fact that it can take 2 hours to get a DF PUG for these hard parts of the game tells me that you're barking up the wrong tree, and making the storyline optional would just have people do the dungeon content once to clear it and then never again unless they want the full gear collection from that dungeon or are trying to do the relic weapon quests.