That's because the carrot to doing this content is something that has persistence.
Glamour gets rid of any reason to even get gear for it's appearance. I'd love to keep using the Relic or Anima weapon if it just grew with the leveling, but when you can just throw away this hard-earned weapon with the next patch level's vendor trash gear, there is something wrong.
Really the kind of Horizontal progression I want already exists. Here's your job, if you want to play another job, you can farm the gear for it, or buy it, or whatever. You still have to level it. Other games don't offer this, and in games that have anything similar, you usually can't do everything without cash shop resets and grinding on a different skill tree.
Where I think other games make a mistake:
(Prefix) GEAR NAME of (Suffix) + (Bonus multiplier)
Rarity
GEAR NAME
Durability
Base damage/defense
Attributes (eg cold/poison/fire/thunder) resistance/buff
Modifications (eg materia)
So you're trying to use the same weapon, and apply 5 different modifiers, while risking destroying the gear or modifications. The most aggrivating version of this I saw in both Mabinogi and in Wizardy. Both games you can destroy the gear by applying the prefix/suffix blessings/curses, and you can destroy it by the incompentent repair guy with a 90% (more like 30%) repair rate, and you can destroy it by modifications. You have less than a 2% chance of getting all these things right. Then there's dying which is another RNG.
Like I enjoy the complexity to a point, but not when RNG is involved. FFXIV removes this complexity down to a single thing, the materia. For all intents a NQ is +0 and a HQ is +1 in terms of other games descriptions, and the blue/green/pink gear rarity is completely meaningless.
What I'd like is that gear meant for one objective (eg MSQ, PvP, Raiding) have it's own horizontal progression bonus that otherwise is not relevant to the other gear. So PvP and Raider gear is different, they're both ilevel 375, but the raider gear would be better for raiding, hence why it's rewarded from raiding, where as PvP gear is better for PvP and is rewarded from PvP. If you go back and play MSQ content with any of those gear's, it's still ilevel 375 gear, just it's bonus may not have any practical use in the MSQ content.
Let's say for example, that you can apply a PvP buff prefix, and a raider buff suffix to the gear, any gear. This buff is permanent much like materia, but it's only active in that content. That gives the player an incentive to play that content on their main to acquire the gear most relevant to playing that content, while not hobbling them in any other content. Especially since MSQ content is relatively easier from the overgeared ilevel alone.
The existing system actually can support this by having a "prefix" and "suffix" materia slot that applies a permanent buff or debuff. The buff's don't necessarily make the fight easier (the buff might just be cosmetic sparkles and rainbows) but you could have things like float permanently applied to nullify one mechanic that otherwise is a KO.
Switching gear between mechanics should expressly not be a thing.