Need to address the Eureka in the room.

[purgatori]

What's the point of a massive world in an MMO if it's empty and almost everyone is just doing instanced duties.

Well, that's certainly a broader definition of 'group' than I am accustomed to. And if one were to equate that definition with what it means for a game to be an MMO, then I have less issue with that, as I feel it encompasses both those players who do like to be in traditional groups, as well as those who like to work towards a common goal with other players, but not necessarily be in a party with them.

However, I also don't think an MMO would suddenly not become an MMO if it also had a large amount of content wherein there was very minimal interaction with other players. And while I don't think that Dark Souls could be construed as being an MMO, I think that if it were simply more 'massive,' it could potentially be made into one. To me, even interacting with an economy that is affected by other players counts as being a 'multiplayer' activity, so I really have no issue with the MMO definition so long as some minimal amount of multiplayer interaction is required of the players therein. Of course that's just my take on it: the primary appeal of an MMO has always been the 'Massive' part of that particular initialism, and also that the 'RPG' part is typically implied even when omitted from being printed in said initialism. For me, multiplayer counts for very little, and what keeps me coming back is that the content that my character can progress through is always being expanded. I really don't like it when I get to the point of having done all the things there are to do in games like Dragon's Dogma or Fallout: New Vegas; if those games were updated with new story content year after year, I doubt that I would bother with MMOs at all, to be honest.

I agree with you that there's no point to the open world if nobody is there participating in it, but I have serious doubts that this is because '90% of the contents' are soloable. I have only to point to the ever-populous zones of ESO, which are also at least 90% soloable. But what ESO's zones have over FFXIV's are a wealth of side-story/quest content, rewarding loot, and activities that remain relevant even at endgame. If I could progress towards a relic weapon by grinding FATEs/mobs in open world zones, and then joining up every now and then to take on a variety of world bosses, I would; but grouping for challenge logs is absolutely tedious, and not my idea of fun. The group play side could still be preserved---especially considering your definition of what it means to group---in that the bosses would still require player cooperation, but players would not have to organise and maintain groups just to engage in activities that should be relatively trivial for a solo player to engage in (as they were in the old school MMOs I used to play).