@Wagomu:
I can certainly blame Nick for only ordering the initial set, because they didn't do that with the original ATLA, when it was much more of a risk and had no built-in audience.
Nick only ordered a partial first season of the original series initially, not even the entire season of 20, and the show didn't then try to cram its entire story into 12 episodes regardless and tie up everything in the literal last minute of episode 12.
Yes, Avatar was a success, but Nick never knew what to do with it. It being a sequential series messed with them, (they just wanted to air interchangable Spongebob style shows) and they had no place to put an action adventure show and they always mishandled its scheduling as a result. (Giant multi month hiatuses followed by blowing through the last 8 episodes in a week.) Audiences that came in for Avatar didn't stay on the channel for anything else. The situation might be different now that they would have had Ninja Turtles to pair it with, but… Avatar was easily the best show Nick has ever made (followed by Zim a decade earlier) but it wasn't exactly a good fit on Nick.
Nick absolutely deserves a chunk of blame for not going "You're a proven success, we'll give you 50 episodes over 4 years" or at the very least, not saying a couple months into the first season "Okay, we'll give you a season 2." But like I said, I can't fault them for only ordering 12 initially because that's what they did the first time around (or maybe it was 10 or 14, I don't know the exact number) but something in the pipeline screwed up the resolution and follow up. The creators dropped the ball on the ending, and season 2 definitely suffered for it.
The main thing is, other series, animation and live action, get by on season by season orders with no guarantee of renewal and they manage fine and pace themselves and not having a super long term order isn't an issue, they do it season by season. (except when they end on cliffhangers with no guarantee of follow up. Screw that.) The creatives DO deserve some of the blame for dropping the ball. THey are the ones that thought spending 1/3 of the first season on pro-bending instead of other things was a good idea.
The first season of Korra was written ONLY by Mike and Bryan, whereas the original series had a ton of writers and directors involved, and that's clearly a large factor in what the difference ended up being.
With Korra it's mostly frustrating because its super duper extra clear where the lines fall, and it takes only a teeeny tiny bit of imagination to see how something as simple as just moving a few pieces forward or back as little as two or three episodes, or where making a cut on something, or a slightly stronger grasp of the scope of the whole piece, would have made all the difference in the world for the quality of the entire series as a whole.