Ok, to correct a couple things.
They really wanna make Korra look good by comparison.
Speaking of Korra, I hold the view most of the problems with it was he constant lack of a timetable and over arching story. Every season was "the last season" and production was constantly being done with that in mind. Every season ended up as a single story instead of the big arch that the Original Series had. This, above everything, killed the show.
Everything else that happened, with Mary Sue vibes from Korra to virtue signal SJW themes, can be squarely blamed on the lack of timetable and constant management refusal to commit to a series that was at least 30 episodes long and planning for seasons in advance. Every problem with the show could have been drastically reduced or even eliminated had that been done. Had the writers been give a timetable by management saying "We are gonna make 4 seasons of 12 episodes" or even "We are gonna fund 2 season of 12 episodes each, with a possibility for a 3rd season if it does well" the writers and animators would have had better time and the show's quality would have been much higher.
But, as we all know, the higher ups and suits rarely have common sense or understand the work the underlings do. They didn't understand what made TLA good and what made LOK bad. Monkeys throwing shit at a wall to see what sticks without any sort of rhyme or reason.
Bear Ribs is right, every cartoon, every TV show really, has to deal with this. If you are making an original TV show with a multi-season story arc not based on novels or anything, it's very rare to get the whole series guaranteed from the start. Most shows are written without assurance that there will be a next season, ATLA included. The initial production order for ATLA was actually just 13 episodes. It eventually got expanded to 20 episodes, but that was after the first 13 episodes were written. They wrote the episode "The Blue Spirit" under the distinct possibility that it would be the last episode they got to make. They chose to write the story with a longer story arc
in spite of that possiblity.
With Korra, I don't think anything was stopping them from writing the same sort of overarching story. They chose not to. Korra was initially conceived of as a 12 episode miniseries, and expanded with season 2's story and then the stories of seasons 3 and 4 (which were announced together and do seem to have been written together). You may prefer the creative choices they made with ATLA, and feel disappointed with LOK. But it is in fact the result of the
creators' choices, under similar conditions as ATLA (if not better conditions, as the creators have said that Nick approached them and said they could basically make whatever new series they wanted as long as it was in the Avatar setting). It was not the big bad corporation choking the creativity of the writers.
Also. I've always found the suggestion that Korra is a Mary Sue to be baffling. Like...no. She specifically goes against many of the defining characteristics of Mary Sue, especially in season 2.
The timetable thing is just an excuse.
All cartoons live with not knowing if the current season will be their last,
Avatar: The Last Airbender included. In fact, A:TLA itself was at one point plotted out to have
four seasons and got cancelled after three so they never got their four elements deal finished and wrapped things up at the end of season 3 instead. Many better cartoons from
Gargoyles to MLP the entire DCAU managed without some guarantee of how many seasons they'd get ahead of time.
I'm in general agreement with
@Val the Moofia Boss. I wound up dropping Korra after the first few episodes because I strongly disliked Korra as a character (egotistical, way "cooler and more powerful" than Aang, disrespectful, undisciplined) and didn't find they were adding any other characters I wanted to watch, certainly none worth having to put up with looking at Korra for.
The idea that ATLA was going to have a fourth season is fake news. I've been following the show since the start. The co creators were always very clear that the show
was not cancelled, that they believed a story should have a defined beginning, middle, and end, and season 3 was where they had always intended to end it. There was no "four elements deal", each season was named for the element Aang learns therein. Aang started the series already knowing airbending. There was never going to be a "Book 4: Air" for ATLA.
(Time stamp at 24:15 for relevant comments from Bryan Konietzko, one of the creators)
In recent years Aaron Ehasz, the head writer for ATLA, has talked about how
he wanted a fourth season, the ideas
he had for a fourth season. But even though Ehasz was a very influential writer on the show and contributed a lot, ultimately he was never in charge, it never would have been up to him. What's egregious about the article you link to is that it acknowledges that Bryan Konietzko has specifically denied that there ever was going to be a season 4, but pushes the narrative that there was anyways. Fake news.