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Top 10 2d mmos: http://mmohuts.com/editorials/top-10-2d-mmorpgs
Half of them have shut down! Why's that? If anyone plays these.
Most of them were a little too childish for me. MapleStory was fun for a few hours.
I really liked the Art direction and overall mood (including music) of Ghost Online. I think sidescrolling MMORPGs are a great way for independent developers to create unique games without having to have an army of programmers. Dragon Saga was also good, but got boring really fast.
I think there are still a lot of players who enjoy 2D. It's just that these games are often Pay2win and/or full of bots. But on the other hand, most f2p games are. I have problems finding a single MMO where I can just pay the monthly fee and just be left without disadvantage against players who are willing to spend money in a cash shop.
Also, not a lot of 2D games were developed lately. All those mentioned in your link are quite old.
There are 2D games which are still doing very well. You should specify what you mean, though. 2D as in 2D platformer? In that case, both Maplestory and Elsword are still performing well, both in NA and Korea.
Those two aren't terrible. MS is just extremely grindy, and the endgame progression is horribly annoying (mastery books). But some people like that kind of grind. There are actually plenty of MMO players out there who are fine just logging in and killing tons of monsters.
Elsword is kinda fun to play. Combat certainly is far better than MS.
A lot of the 2D platforming MMO's which failed or achieved only marginal success suffered because they were only marginal improvements over Maplestory.
And there are still tons of new 2D games being developed.
They're all just browser games and not side scrollers. Although, to be quite honest, even this will be obsolete very soon. Not only do we have excellent 3D browser mmo's now, but we're also getting excellent 3D mobile MMO's.
Kritika is on fire in Korea.