From my FFXI campaign (2003-2010) and casual story grinding here and there, I can easily say that end game was not only some of the most fun I had in MMO's, it was also the most frustrating. Never before had I raged so hard at a game because I do weeks among weeks to months to years to get one piece of gear through content our LS either had to fight RMT, botters, and other legit competitive players. I -never- got my Askar body piece from Nyzul Isle and it still makes me sad to this day. I easily got to the 80th-100th floor of that dungeon at least 20+ times to never see it drop, not even attempt to lot against someone on it. Wait months to see an item finally drop in Dynamis just for someone to beat you in a lot.

I have a hard time believing the audience of this game would be in favor of horizontal since it is the same people who complain that they are bored doing the same content over and over again and we get new content every 3-4 months. Both Horizontal and Vertical have their advantages/disadvantages, but from SE's perspective, which method is going to bring the most people in and keep them there? Vertical is the likely answer.