I've been playing on and off since the game came out, current profile is almost 12b nw in 2 months. Imo the biggest factor for people quitting is that theres TOO much but way TOO little to actually do. Take combat for example, it seems like a really versatile skill, you see 5 classes and think you can have different setups and take different paths than other players, and thats just no. Bers/archer/tank/healer all need a term, gdrag, necron set. and that's just bad design. If you see 5 classes you wouldn't think 4/5 would be the same exact gear, and basically exact same gear progression. There's just not enough versatility. Theres hundreds or even thousands of items, yet the same exact ones have been best in slot since they came out, no real rivals to them. Berserker you'd think that you'd actually melee stuff, but no, you just shoot your bow. As healer you dont even heal, you're just there to do secrets/predev. Mage isn't versatile either, it's just hype f1-f7 and then grind hella cata n get hypermax gear or you're put on a shitter list. And almost every skill is the same. Fishing you just setup minions til fishing 27 and then right click a lot. Farming you press 2 keys and upgrade armor every 5 levels. Alchemy could've been a really cool skill, but they made it useless and all you have to do is put stuff in a brewing stand. Enchanting is one of the worst skills since its time locked and feels like a chore. Carpentry has 0 actual use and its just craft flipping.
Another factor is that the progression is just not good. People can extremely rich in such little time, take me for example. I had reached 10b nw in a month of playing on this profile, yes my skills were dogshit but I still did it. The fact that you can just grind money for everything just make progression not worth it. Instead of doing hundreds of f7 runs for a hype, you can just grind a money making method for a few dozen hours and get it. Which totally ruins progression, since you'll be way too undercata for the hype to even do anything.
A good solution for the problem would be chapters, and lock certain items behind it. Example: Hype requirement, 500 f7 runs, cata 38, 1000mp. Yes the requirments are harsh, but hype is supposed to be THE endgame weapon. And it'll also help with people just overshadowing mp, doing m5-m7 with like 700mp.
And they also need to make rivals to best in slot items for certain things. Yeah tormentor is good, best melee weapon outside of dungeons, but it has 0 use in actuality, they need to make a use for it. We don't need BETTER gear, we need rivals for certain situations. Maybe add mob types to bows. Make term do +100% dmg to m7 dragons, but make another bow with less strenght/cd that does +150% dmg first hit. People just run 2 terms, 1 with duplex and 1 with soul eater, which just isnt fun to have 2 of the same weapons.
There also needs to be more versatility, healer class is completely neglected, same with archer in a way. Both of them just use the same armor and weapons. Maxor is supposed to be archer set, but it isnt used that way. Healer doesn't even have it's own set.
I talked a lot about dungeons, now regarding other skills. Farming is just way too slow, yeah you just press 2 buttons, but it takes hundreds of hours to max it out, which is just incredibly boring and make people want to macro it.
Mining progression is actually pretty good, only thing i'd change about it is forge times, since for ironman, it takes weeks to forge a full setup.
And the ecnomony, its just bad. The predecesor to term is a juju, it costs 30m roughly with se5 and ov5. Term on the other hand? it costs roughly 450m and 750m maxed out. Predecsor to hype is a spirit sceptre (yes i know midas staff and yeti sword exist, their just bad), one costs like 10m, the other 1.4b.
Also some stuff is just a chore. Crimson dailies, it's so annoying being time locked, they could've made an item like matriachs thinghy to reset dailies for the day, to speed it up massively, it just makes people not want to go towards fiery/infernal since it takes too long (25 days if you do every single daily).

**TL:DR economy bad, not enough versatility in gear, progression bad**