We differ on this. It is irrelevant to have to learn your role one skill at a time. On most - if not all - jobs, your Lv50 rotation is completely different from your Lv60 rotation. Meaning once you ding 60 and have finished your Job quests, you finally have all the tools necessary to start with a new rotation. And that will take time to get to know. Even level-sync'd down to interim levels, any player can adapt their rotation based on greyed-out skill buttons. So, regardless of "jump potions," any fresh 50 or 60 is going to perform poorly at their job for the first while, until they are used to all of the new skills ... and build a solid rotation that makes the most of them. And, as such, they will likely need help from others, or play guides or w/e. For example, once I dinged 60 on my BLM, I had a guildie show me how to do a 60 BLM rotation; b'cos, up until that point, the Lv50 rotation was pretty much the best - or at least - good enough.
I'm guessing you haven't heard about SWTOR then? Since the KotFE expac, and the story-driven MMO it has become, it has had boosted characters for purchase that grant both the previous expac's max level, and clears the main story, so you can go straight into the new material: KotFE & KotET. That aside, WoW, Aion, Eve Online, Rift, Guild Wars 2, etc. (just to name a few big ones) all have some kind of character boosting that works for their respective flavours: Boosted characters, XP boost, skill injectors, exp potions, etc. Also, "The extremely long-running MMO Tibia is just now starting to sell experience boosters in its cash shop, 19 years after the game’s release." (source: http://massivelyop.com/2016/08/23/ti...9-years-later/). Just because you don't know about other MMOs and their boosting options, does not invalidate their existence. The point is, they have all jumped on the boosting bandwagon, and FFXIV is no different.
As far as "not know[ing] the story" goes ... I'll just post an excerpt from the OP explaining how that works: "To compare this to a television series, season one is A Realm Reborn, season two is Heavensward, and the newest expansion Stormblood will be season 3. With this potion, you'll be watching the memoirs of season 1 and 2 from an inn, and then actually watching season 3. Naturally you will have to actually watch the latest season, so the short cut will only allow you to skip through to the end of the previous season." Will newbies be completely in the dark on the previous season's stories? It depends... Did they bother to watch the cut-scenes in the inn? Or just jump headlong into the new season? /spoiler: It's their choice.lol
I do agree with you, however, that the "jump potion" does seem a bit like a "band-aid" fix at the moment, and - like you - I can only speculate about future releases (and the challenges for new players), but I can't think of any better alternative that can retain the core structure and values of the game, while allowing new players to catch up quickly to current content. Many other ideas would either break something as a consequence, or require a daunting level of complexity and time involvement by the dev team to create a workaround in the MSQ. Which then begs the question: which would you rather have? A more streamlined approach to MSQ gating? ... OR ... New content delivered on time, all the time? ...Unfortunately, I don't think it's feasible from a business standpoint to have both ... and that's the very real business reality known as opportunity cost.
As far as a "best solution" is concerned, I don't think there is one. Not one that is pragmatic enough from SE's shareholder's perspective that is. Ultimately, it's about business, managing this game, its players, its future, and new content - all as one big juggling act that we, as subscribers, cannot fathom.
Zero-to-Hero / "jump potions" are a paid thing now, but maybe in the future this will be made free. ... And we can only hope with baited breath as to how it will evolve in 5.0 and beyond.