Video Game x Brain
I believe the greatest reason I have come across to not play games like Maplestory and to instead play other casual 3d games is that addiction to videogames and too much videogame playing may lead to poor health and decreased function of the brain, something I wouldn't personally want since it affects socialization, academics, memory, whereas casual 3d game playing that requires somewhat difficult hand-eye coordination and strategy in a 3d environment may lead to increased brain function. Maplestory is 2d and does not require much hand-eye coordination at all, and it is made so you would do repetitive tasks for hours on end to get through it. It seems like one of the most unhealthy games for the brain. Whereas you have other 3d videogames that are simultaneously harder and easier to play casually (30 minutes to an hour). You can imagine that games like chess, overwatch, call of duty, starcraft, perhaps card games even like hearthstone, are more rewarding and easier to play within a 30 minute to 1 hour time frame since it pits you against opponents within your skill level (allowing winning), and also are more interactive tasks for more parts of the brain. You just have to limit your playing. Maplestory and games like it that focus on grinding for gear are more difficult to feel rewarded in since you will have to grind multiple things to get ahead, and to go through it at a nice pace for free would require hours of gameplay, which I have suggested would negatively affect you. That's why I would recommend thinking about this and looking into other games and limiting gameplay, unless of course you don't care.
Also -- and I believe it would be remiss to not say this -- from my personal experience, when I have played Maplestory for hours, I have felt some type of mental block and adopted a negative attitude, whereas I have not when playing other videogames. Also, when I have read certain classical or technical books that are professionally developed and have a nice flow, fiction or nonfiction, for hours, I have actually felt better and more mentally able to interact with my environment; I would suggest to anyone to force themselves to read at least one to try it out. I have also read other works for hours that are pretty low on the reading level which have had almost the same, but less of an impact. Other things like television, comics, and whatnot have had even less of an impact, though there are some that really get me going.