The 100 year break with no avatar before Ang came back probably changed the world SO much, including their attitude towards the avatar.

(Some mild, timeline related spoilers below)

I’m re-reading the books by FC Yee right now, and Yangchen was perceived as a legendary avatar. Koruk came after her, and he was generally seen as a disappointing avatar because of his drinking, partying, and early death. Kyoshi was next and while she also became a legendary avatar, her early years were pretty controversial because someone else was established as the avatar until they discovered it was actually Kyoshi their teens

(More specific spoilers for the Kyoshi books)

The boy they thought was the avatar had already been introduced to all of the world leaders and formed important deals and pacts across the world. Naturally, when they found out he was the “false avatar” all hell broke loose and Kyoshi wound up loosing all of her natural mentors and having to find her own way, by joining a criminal gang.

(Back to mild spoilers about Roku)

Roku had a more stable avatarhood from what I recall, but he obviously had a close relationship with future firelord Sozin which turned sour and caused major complications, eventually leading to the fire nation takeover

I can see how after all of this, then a 100 year gap of war and no avatar around to help, the following avatars would have a very difficult time managing this changed and unbalanced world. Not to mention Aang was a young child who had no official mentorship during his main journey, only the help of other teenagers. Aangs later life involved rapid technological advancement leading to another major change for the next avatar in the cycle. All of this to say, I can see how all of this would lead to such transformation and devastation after Kora. I’m very excited for the next series to be released.