Not sure how it was any better in FFXI... seemed like half the classes had to band together to form their own parties because nobody wanted them.
Every time I read about how horizontal progression works, it seems to come down to having 100 different ways to augment one class so there's no gear obsoletion and 100 special snowflake combinations. That's cute and all, but the playerbase will simply find the most optimized option and throw the remaining 99 ways in the garbage and refuse to play with you if you use any of the discarded combinations.
Even if they threw in being able to swap gear even in the middle of battle, then the playerbase would find the most optimized 4-5 of them, and throw the remainder by the way side. Everyone would simply look up the recipe for the best cookie cutter. Even if, on top of that, you threw in them rebalancing the game every month, aka flavor of the month, you're only making it harder for those who don't want to min/max and the elitists will only thrive all the more.
In the end you wind up back where we are now, only with a bunch of wasted development, and very likely a playerbase that has been refined down to the players willing to jump through the hoops to continue playing as they're playing right now.