I don't see the connection. Vanguard wasn't exactly hard core compared to EQ and it had so many other issues technically and on the management side that I think it's dicey to try and draw a conclusion.
Massively said in a review of ESO recently that they predicted it will flop because people are tired of "story-driven theme-park MMOs." I thought that was an interesting statement. I would certainly be in that category of players, but I don't know if it's the majority. I like online fantasy RPGs in some ways more than single-player, even though the latter are richer in story, but they have become just such utter trash that I don't even consider the possibility that one will be good anymore. They've become literally the archetype of boring. Just follow little pointers around the map and click shit. It's nuts. I'd love to think someone will do a big, gorgeous, open world where you can just get tossed in and explore, fight, loot, and die, but I have no actual hope it will happen. I think it's over.
Well, the comparison is that that's what Vanguard was supposed to be, and so it sends a message to people who decide what to make.
I played EQ vanilla, and I remember the experience was filled with the game having so many nasty things it strengthened the community that needed to cooperate, and I remember thinking that the frustrations were so high that people would tolerate it because it was the only great game of the type, but I felt when someone made an alternative that was less frustrating, people would often prefer it and competition would force mmo's to get less and less hard. Wow closely fit what I had predicted and the rest is history.
Despite the name, questing wasn't the main gameplay in the original Everquest - it did have a lot of 'pick your own gameplay'. But that also did lend itself to players quickly feeling they were 'done' with an area other than grinding in it. For example, farming giants for plat, or spots for experience.
I did notice that left to their own playing, players could find a new place, run around, kill a few things, and in a half hour feel 'they did it, done with that'. While questing has a way of extending content use greatly - suddenly it's kill 12 of this and 10 of that, collect 15 of those, and all the stuff in the area is used for up to hours and hours. And it can help players re-use content and feel less 'done' so quickly. And the story side of the quests helps with that even more.
In EQ, players made the convention that the East Commons Tunnel was for player auctions, while current MMO's have the far more functional Auction House. Each has its pros and cons. But my sense is that most players don't prefer the sandbox atmosphere, even if it was accepted when it was the only game you could get of the type (of course it was just 'more' sandbox, not really sandbox).
A lot of the MMO conventions now in use - or overuse - I think are because they work pretty well for players, despite the things you don't like about them.
Part of it does come down to the players. It's like a game naming a server the role-playing server - what the players use it for has a lot to do with whether that means much.
MMO's seem originally to have been more about 'provide a world to explore and spend time with players in', while that's gradually evolved to 'provide a world to have players follow story quests and get advancement in'. EQ offered some huge challenges that seem mostly rewarding just for beating them, while now it's more rewards for completing things that don't take as much nearly to get done. But players mostly seem to prefer that. EQ was amazing but it can be pleasant to have the softer gameplay.
An example I use of the craziness of some EQ design was a man and wife couple who took 2 weeks vacation to take turns sleeping and camp her cleric 24 hours a day for over a week for an epic quest piece. Some things could demand staying up for over 24 hours. Another example is the original zone where you couldn't leave without fighting out, and if you didn't, you permanently lost the equipment you had spent months to obtain, and if you got killed, you had little way to have the equipment to go back in and get back to your body.
That's too nasty, I only remember one guild trying it, so I think it didn't get used much until they relaxed that.
Anyway, I'm not sure how much market there is for the more 'sandbox' you mention, and players don't seem to be used to that idea anymore.
Player tastes change. I met the guy who co-wrote Zork in EQ, and asked him, doesn't he think more games like Infocom did could be made? I was more optimistic there was a market than he was, he flat out said there's no way for that sort of game to sell anymore. Despite how many say how much they liked them, apparently it's not what people buy.
The whole 'free to play' design similarly is a case where 'the market has spoken', despite all the criticisms we can make of that design over subscription games.