For the first question, I think we wouldn’t look at ATLA the same way. Avatar is so loved partly because of its consistency (think of the reception of its sequel, Legend of Korra. That show is more disliked, though it is still held in high regard, just not at ATLA levels).I appreciate the superlatives and don't think they're hyperbole.
But I am curious - if the series went past 3 seasons, and they weren't the same quality as the first 3, would we look at it the same?
Or put another way, if SpongeBob ended at 3 seasons, would it have the same critical consensus Avatar enjoys?