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I dunno, I think you might mean the combination of dramatic-action cartoons. Though they aren't particularly a dying breed; they're just not as popular, since drama-action tends to be more serialized, which is a no-no for networks, and target an older demographic, which may not be so interested in toys.
In the case of Korra, this news, while disappointing, shouldn't be so riot-inducing. Nick basically gave up on getting the target demographic so they're tossing the episodes online to the other, older demographic which is quite possibly greater.
You know not all Drama action shows are multi part, Samurai Jack and BTAS were episodic.