I think the term of immortality is not exist. But instead Kaguya can be like Madara feed on human to live on.
And her mentioned threat is not really about stronger enemies, but more like those enemies has the same method to live on like her and Madara. Just like food resource being divided and that sound more danger to Kaguya life rather than a stronger opponent. When you have nothin left to feed you simply die.
Madara version was inferior (maybe) since the tree is smaller and less target, also Madara awakened his rinnegan when he was almost died because of old age, so that explain why he looks so old.
And her mentioned threat is not really about stronger enemies, but more like those enemies has the same method to live on like her and Madara. Just like food resource being divided and that sound more danger to Kaguya life rather than a stronger opponent. When you have nothin left to feed you simply die.
Madara version was inferior (maybe) since the tree is smaller and less target, also Madara awakened his rinnegan when he was almost died because of old age, so that explain why he looks so old.