First, FFXIV's "progression" is basically "gear, gear gear", which in other "RNG-only" based games (see the the 2.x raids) means you may have to play the same dungeon 1000's of times to get the piece of gear you want. That is bad. It increases player fatigue, and when people burn out on a game, they don't come back. That's the entire addiction problem in a nutshell. When you erase that person's effort by releasing new RNG gear that obsoletes the previous RNG gear, then people just quit, because they don't want to commit that much time, and probably subscription/cashshop to it.
Loot Boxes, is basically this, but without the game play. Why bother creating any content, just put the gear into gachapon, and let peoples gambling addiction take over.
Which, means, that "RNG" based reward gameplay is bad. Think about the difference between farming Atma's for the relic weapon, and farming these crystals to upgrade gear that you already paid tomestones for. At least you're gauganteed one crystal IF you get a chain of, what is it, 30? Let's do the math, if it takes at least 30 seconds to kill one enemy x 30 it takes 15 minutes per crystal, and you need 100 of them to upgrade one item. so you're looking at a minimum of a 25 hour grind provided you get no other crystals, provided you can actually maintain the chain. At least it's not like the Atmas which you can spend a week doing FATE's in Southern Thanalan and get nothing. Likewise when people were doing Coil, you could only get one reward per week, thus if that thing you wanted didn't drop, nobody was going to do the raid again just for you, if they got their reward already.
Hence, the "coins" and "parts" and "tomes", and "Crystals" are a way to ensure there is always some progression, you're never actually wasting your time (in theory)
In practice, people don't like the pace. Yeah, maybe it should not be such a grind, because it puts off solo-play, but we're not looking at the big picture either. What other way can people get rewards that are not "replace old gear with new over powered gear that makes high end content a faceroll?", there's all that level-1 glamour gear, which works for costumes, but still, think bigger.
What do other games do for progression? Advance other skills, that aren't used in combat. Like, I've not done much of the crafting stuff because it, itself is a tedious grind, what's the ultimate reward from that? In other games, crafting tends to be entirely RNG as well, and tends to not advance the game in any shape. Crafted HQ gear should be equal to whatever "end game" gear is present, otherwise there would be no point to crafting anything at all, but the gist of what I've seen is that crafted gear is entirely pointless. Blue gear has one or two materia slots. But this Eureka gear has 5. Yet is still not BiS. No, the BiS gear is the gear you get from turning in the Sigmascape datalogs.
So there is already quite a bit of grind in the game, regardless. It's either via tome farming, crafting, or playing the hardest content. To quote a friend who quit playing a different game "It's just a geargrind, there's no point."
At least with every piece of content we get, there is storyline that goes with it. Other games tend to just release new gachapon, and that's it.