Let's just take a moment to compare inventory costs (the average amount of inventory necessary to play the game) between 1.18 and up until 3.0, then.

1.18 had no class-specific gear except by armor class. Many armor classes now locked to specific classes were even then still open to classes meant to play at lower armor classes, at reduced stat values.
1.18 allowed you to use gear above your class level, albeit at reduced stat values.
1.18 had no job gear required.
1.18 had no need for glamour base items or glamour catalysts. (These systems had not yet been added.)
1.18 had fewer dies.
1.18 had a loot-list to facilitate the filtration of inventory intake and transfer of loot among the party.
1.18 gained a larger portion of stats from levels than gear.

2.x added armory chest slots, but also:
- Increased gear dependence (less portion from base stats)
- Increased gear in the game
- Removed the "optimal" system which allowed the use of above-level gear and use of gear from other armor classes
- Added additional job-exclusive gear
- Increased the number of gear tiers over patch cycles
- Allowed for glamours at glamourable items +5 catalysts' per type (Goldsmithing G1, G2, G3, G4, G5, Smithing G1, G2, G3... etc.) slot cost.
- Added two classes and three jobs

Since then we've condensed glamour prisms into a single grade of each type and added additional armory chest space, but have also added 5 more jobs and numerous more glamourable sets.

Regardless, then, of whether one is intended to "hoard" items, consider: why was the glamour system made in this manner in the first place, rather than reducing the data necessary to serve the same function into hollow tags alike to achievements? Why, if one is intended to be able to level each class or job at their own pace are there still barely more armory slots than total classes and jobs (keeping in mind that unless all classes/jobs that share gear are leveled simultaneously, all gear between the lowest and highest level classes/jobs in that category should be saved, even if glamour weren't a thing)?

Just in Stormblood alone, there are 70 items required to level every battle class, or 80 to level truly every class, per gear slot (with rings duplicating, but 205 mainhands and 65 offhands in total). Compare that to our 35 armory slots. In total, that's 1005 pieces of gear to have the highest ilvl gear available at each level when leveling, if initially unable to get higher than i255 at level 60, every battle, gathering, and crafting class. Belts and other items not desirable for glamour can be freely removed once all classes sharing that gear have been leveled, but that's still a ton of gear that many may wish to keep.

1005 slots just for leveling everything, just covering from 60-69. No food, no mats. Much of it may be desired for glamouring later.

Does our current inventory space of 525 still seem massive by comparison? It is, admittedly, a massive amount, no doubt. But so too are the inventory costs.

Compare that to the up to 136 on-hand and 220 bank space available per one combat class and two professions on WoW, who do not need to hold onto their inventory for glamour or shared-gear classes (the items having already been bound unless BtA), and who can always mail excess items to up to ~55 other characters per battlegroup (datacenter) for an additional up to 356 slots each. I feel like I have a lot more inventory space, relatively speaking, over there.