To be fair, this game has always advertised itself in promotional material as "8 paths, 8 stories to be told". The developers probably led people astray by mentioning Final Fantasy 6. It's clear that this game captures the same aesthetic as that game, but tries to take a very different and non-conventional approach to the storytelling.
I'm a little bit concerned that all the hoopla about Final Fantasy 6 means that Octopath Traveler is not going to be judged on the merits of its own design and storytelling decisions. If I read a review and someone complains about a lack of an overarching narrative, I'm actually going to roll my eyes. Does it have to have an overarching narrative and intense character interactions to be a classic RPG? Of course not! In fact, thank goodness the developers are trying something different.
I'm not interested in someone complaining about something that isn't there. What I really want to know as a consumer is:
a) Are the eight stories actually good?
b) Do these design and storytelling decisions made by the developers actually work?
b) Do they add or detract from the overall game?
If the developers did not successfully execute their own design decisions, then criticising it is fair enough.
One other note - I've probably read about ten previews of this game, from US, European and Japanese authors (translations). For some reason the American previewers are by far the loudest in complaining about a lack of an overarching narrative. Most of the European previews I've read have either not mentioned it, or only mentioned it in passing and said it wasn't detracting from their experience. .