I think there are different reasons why players are quit, because this game have totally different playstyle at different phase.
- Unfriendly Progress
Some players left during <lv50-100, because over-questing failed them the impression.
Some players left during <lv160, because 115/130/145 dungeon is very exhausting and then suddenly stuck at 160 that too lazy to grind.
More players left during lv230, when they reached R7 with some skill points to test out the class and then lazy to grind and find party, mercenary getting too boring.
More and more players left during lv280, because they feel nothing to progress and too lazy to retrain a new character to suffer over-questing.
To summarize, unfriendly progressing have been part of the big issue that this game is non re-playable. To suggestion, I think something like Grynas Event / Storage Event need to implement more since low level and of course with more innovative idea and not only always hour:00, I like GW2 random events tell story through live responses but that was too much I think, in ToS 1 in 5 maps and spawn random in 1-2 hours will good that grant luxurious exp. It unite players and give the world more lively.
Saalus Convent mission need to grant EXP to replace Mercenary, and better to push earlier into 230, since 230-240 is a quitting spot for many players, and the items need able to transfer.
- Class Imbalance
Not to mention many important adjustment are not implemented iToS at all after 3.5 months, I feel IMC is either too careful on this or too ignorant of this issue that plagued the gameplay a lot.
Those bugs and new content delay need to address, but players can just play with keep an eye close.
Market restriction is double edge sword, it is still ok despite seeing most items price is quite stable, but some min-max price really need to be fixed.
Man, IMC really need to increase their resource to tackle these issues quickly. This is a best time where there are no other decent MMORPG coming, it also could be MMORPG is actually dying and not many company want to do that.