I watched atla as a kid and I ended up liking korra more, I just found korra to be more relatable, she feels really human and deals with real trials of life. Aang feels like a disconnected fantasy character, I love him, but people don't act like him in real life so he's very hard to relate to (outside of the Katara romance which was well done imo). I think it's less about nostalgia and more about people who are introspective. Everyone I know that's done alot of work on themselves and dives deep into philosophy and politics loves korra. It's a classic star trek vs star wars debate, star wars is way more fun, like aang, but trek is more intelligent and better written like korra.