Those aren't true horizontal progression, though. That's vertical progression with more useless pieces of gear. If you have 12 gear slots and 144 pieces of gear to choose from that are all the "same level" and specific stat caps to reach, you're still only going to pick 12 of those 144 items every time, and the other 132 are either going to hit the floor or take up space in your inventory.
True horizontal progression has situational gear, such as DPS sets for tanks, healers, and support, resist sets for everyone, damage type sets, different emphases for different resists, etc.
XI is the crowning example of how horizontal progression works, and also how it's a horrible idea for games that aren't FFXI.
For endgame gear, there are currently about 5-6 different sets of iLevel 119 gear in the game: Relic, AF, Empy, and 2-3 more I'm not up on, plus tons of jewelry.
Many of those pieces augment specific stats or skills, and you can change equipment in combat so people will actually macro gear changes to coincide with specific attacks (and since your character briefly blinks out as the equipment is applied, this means endgame raiding looks like a strobe effect).
Unfortunately this ALSO means each job uses up an absolutely absurd amount of equipment slots, and since this is one of the few games where your worn equipment takes up inventory space, you're already limited. Now, if this were a single class structured game it'd be fine, but XI uses the Job system similar to XIV's, with the addition of subjobs. So without a lot of inventory finagling and mules (that you have to pay more per month to have access to) the idea of having more than one job "maxed" means it takes about 5 full minutes to swap jobs as you need to completely change your inventory loadout. Certain mods help this but they're...kinda gray market.
If you go horizontal, you NEED multiple sets to justify going horizontal. Otherwise it's just vertical with more useless inventory garbage.
The only other game I can remember that did horizontal progression was EverQuest 1 (and probably other games of that era), and that was completely nullified in recent years with the introduction of Defiant gear that was used to streamline the leveling process. Other than that, most "horizontal progression" left in the game is getting random knick-knacks to help with the latest set of raids.
And glamour is not a reason for horizontal progression and shouldn't even be considered as an excuse.
Ummm...that exact situation already plays out currently, except healers don't really NEED extra gear options to go DPS since crit and det are used for both and Cleric Stance trades MND with INT.
Which is part of the issue with adding horizontal to this game: We have consolidated stats. We don't have "Cure Potency" or "X skill increase," we have the main stats, det, crit, parry, block, accuracy, skill speed, and spell speed. With the exception of Parry, Skill Speed, and Spell Speed, the skills are all universal against all jobs, which makes making diverse gear problematic.