I can see where you're coming from, but if you just come up with a new relatively-isolated story arc still involving the established cast... well, at that point, isn't that basically Shadowbringers? Even though the story stands on its own a lot more than Heavensward (for instance) did, it still builds on all the things that came before. People who story-jump right to ShB and still want to enjoy the story are like, "Wait, who is this person? Why am I supposed to care?" And the impact of things you learn lore-wise is greater because of all the previous history with the Ascians and how Hydaelyn and Zodiark's conflict has been portrayed previously.
I'd argue that even 2.0—itself a 'let's start a new saga'—suffers from this a bit; I've had friends start playing recently who are like, "I get that this 'Louisoix' person was important somehow, but what the heck did he do? What was this 'Circle of Knowing' anyway?" or "Who the hell was this 'Nael van Darnus'?" and so on; folks familiar with the 1.0 storyline, meanwhile, have the context.
This isn't to say you can't find ways to mitigate that, but it takes a not-insignificant amount of time. And to separate it out entirely—to get rid of all our current characters like the Scions, Raubahn, Pipin, the Elder Seedseer, Aymeric, Cid, Wedge, Biggs, Nero, Shadowhunter, Estinien, etc.—feels to me like at that point it's less "the next expansion" and more like when a successful television show turns into a franchise and cancels one show while a new one launches. Sure, they took place in the same universe, but not everyone who liked "Star Trek: the Next Generation" moved on to "Star Trek: Deep Space 9". And not everyone who loved DS9 moved on to Voyager. Not everyone who enjoyed Voyager moved on to Enterprise. And no one enjoyed Enterprise.
(I kid. Mostly.)
Plenty of people played the Secret World despite the objectively broken combat system solely because we enjoyed the story, and I have no doubt there's a similar chunk of FFXIV players who play the game solely for the story and don't care much about savage endgame or fine-tuning their combat rotation. (And then there's people who care about both things, like me.) We have no way to know what percentage of the playerbase that represents; I don't know that even SquareEnix necessarily knows. And we don't know what SquareEnix's target playerbase size is; sometimes you don't want a game to expand too much beyond a target level because the resources necessary to maintain the playerbase become too high.
(Presumably SquareEnix does know their target size. Or else they need to have serious words with some department of the company.)
That said, I do think that catering too heavily to any one niche of a game's playerbase at the expense of others is unwise. If alienating the story-driven playerbase is almost certainly a bad idea for a game like this, I don't think alienating the endgame chasers and hardcore raiders is a great idea either; I'd argue that the in-game economy around any given patch is driven by those chasing the new glamour items (witness how much the Rebel Coat went for when 5.1 hit, and how much the Dalmascan stuff is going for right now) and those chasing gear for raiding (witness how much the Neo-Ishgardian stuff went for initially). But catering too heavily to one smaller target, such as making your entire focus the diehard raiders...
Well, as I said in another thread, press F to pay respects to WildStar.
If the playerbase is at or near the target size right now, and 65% (hypothetical number) of the playerbase are casual players who are here for the story 30% are diehard raiders and endgame chasers solely here for endgame content who don't care about the MSQ, while 5% are here solely to gather/craft and play triple triad... well, if you restart the story—effectively launching a new 'show'—maybe not all of those 65% follow you. I know a lot of people—me included—who eventually drifted away from SWTOR after they basically went "Okay, and now... entirely new story nearly entirely unrelated to all previous story! *jazzhands*" (Despite the fact that I really did enjoy the addition of Theron and Lana to the companion group.)
If the playerbase is way below their desired the target size... well, then maybe it makes sense for them to examine options to get more folks in who don't care about the story.
But without access to numbers that only SquareEnix has—assuming anyone has an accurate census of the playerbase's interests and motives for sticking around—I expect it's really hard for anyone to know whether a massive shift in story like that would be a wise decision or a really ill-advised one. So I imagine that for now, carrying the story onwards (for the story-focused folks) while providing jump potions (for those who want to hop right into the content) is their way of hedging their bets.