There are pros and cons to both sides, but I personally prefer horizontal growth by a longshot. I could never stand WoW's constant updates, especially when they're so extreme. Going from 80 to 85 essentially made your character 5x stronger. That's completely ridiculous and it does invalidate everything else the developpers have worked on before. If it wasn't for the achievement system, most people wouldn't bother running any of these old instances ever again. The people who run them for leveling purposes are usually just leeching off a higher character and not experiencing them as they were meant to be.

It's not like everything is perfect for WoW either. I remember reading a few months ago that their numbers we're down significantly since the launch of Cataclysm, because there was not enough content, and whatever there was was way too easy to accomplish. I quit about two weeks after it came out, and my Druid was raid ready, had all his professions to 525 including Archeology, had all the quests of every new zones completed and was essentially out of new content outside of the raids, which I feel no reason to do as everything they drop will be obsolete a few months later when they release a new update.

I much prefer a system that doesn't obsolete everything else. Sky still had plenty of relevant gear by the time Abyssea came out. It might not have been the "cream of the crop" in many cases, but that didn't make it completely worthless to anyone who didn't have hundreds of hours to pour into HNM camping or Salvage. Byakko's Haidate, Kirin's Osode, Wyrm Leggings, Wyrm Gloves, Shura Togi, etc all still had very regular uses for many classes. It might not have been "hardcore" to do for people deep into endgame like I was, but I still enjoyed going back and helping more casual friends obtain their gear and showing them the ropes of endgame.

Plus every expansion brought with it new zones and content designed not only for hardcore endgame players, but for people still in the process of leveling and for casual players as well. I never felt like the game was stagnating, it always felt like it was ever expanding, and even leveling new jobs brought new experiences, as old camps got replaced by new ones. But even those older "obsolete" camps were still viable options for those who wished to avoid camping on top of 4 other parties and who didn't mind a challenge that no colibri could ever offer.

I do think they made mistakes. I never liked the melee-burning trend that TOAU brought. I wish they would have found ways to expand crafting as well, outside of releasing new ingredients and recipes once in a while. Synergy always felt too complicated and mostly pointless. And I hate Abyssea and the Trial of the Magians path the developpers chose to go with, but it was a blessing for more casual players I guess, even if it ruined the game for me.

I guess my point is that FFXI proved there was no real need for constant level cap raises, that it was possible to keep putting out new content and storylines and challenging bosses without invalidating everything that came before it. Sadly I think XIV is still a long, long way from ever achieving greatness no matter if it goes for horizontal or vertical progression.