Pushing down the old cast to make the new one look better is an old cliché, usually because otherwise it makes you wonder why aren't the previous heroes fixing everything. Granted, there's spite involved here too.
The world of ATLA is perfectly built to cause this problem tbh. The avatar has to die for someone else to inherit the position, and supposedly they're supposed to find out they're the avatar at age 16, though Aang found out at 12 because of the war and Katara found out as a toddler because SHE'S SO QUIRKY AND SPECIAL!!
Something else that I
wish I could find the video I heard it from, is that every avatar ultimately has to deal with the shit their last incarnation didn't finish. Avatar Kuruk spent his life fighting dark spirits and neglected the tensions of the human kingdoms, so Avatar Kyoshi dealt with it by focusing on protecting the Earth kingdom and creating the gestapo, which made things worse and led Sozin to start the war that Avatar Roku couldn't quash, so Avatar Aang had to deal with that, and because the last 4 generations have been caught up in geopolitical chaos + 100 years while Aang was frozen, the spirit realm was neglected and Avatar Korra had to deal with it. The entire structure and history of the show is that the avatar cannot afford to be human, because they will fuck up and do something or not do something that the next guy has to deal with, which means they will do or not do something else that the guy after them has to deal with, so on and so forth. It's incredibly interesting, historically speaking, but as far as TV show goes it's always gonna result in "whaaaat I thought the last avatar saved the world, why does it need to be saved again??"
Honestly, I'm hesitant to even watch Korra knowing what I know of it. I've heard there are a lot of cool things in it, like bloodbending and metalbending expanding into new users, some guy starting a race war and trying to make the position of Avatar obsolete, yadda yadda, and all of that sounds cool as fuck. But then it's like, "here's Sally McSpecial who's better than everyone by virtue of existing and her woke posse of lesbian brown people come to solve everything in 5 seconds and shit on everything else you cared about." It's like the original ATLA was lightning in a bottle and literally any and every adaptation since has just shit the bed.