I don't know, I never felt like Incredibles had a cliff hanger ending, the Underminer didn't feel threatening, he seemed goofy, it'd be a waste to make a sequel on his account. Whereas The One had Ax, and could have been the set up for a great sequel.
And while you spend like an hour with the Incredible family (or . . . whatever their last names were, I forgot) before the end some people spent years with the Animorphs so you can't really blame them for being more emotional when Animorphs ended.
I agree. The ending of Animorphs was so much more intense, dramatic, meaningful, and emotional. Still, the format of the endings were the same: show how the characters handle the post-war/post-conflict period, then introduce a new threat and have the characters rise to the occasion. It's the same device, on a much, much grander scale.
The answer to the mystery of who/what The One is is that he's a literary device. The ending of Animorphs contains a lot of symbolism and allegory. Tom's voice is never heard, and his nameless, treacherous Yeerk dies in the form of a snake. Visser One, the slavemaster who worked to destroy freedom and free will, is left a blind, deaf slug in a small box. Cassie is freed from a life of war. Rachel never escapes the war. Ax, who called Jake "Prince" and tried to deny himself his vote, is assimilated - but with the potential of being freed by his friends. The renegade Yeerks, whose Empire had exploited the religious tendencies of humans, themselves become a disturbing cult. Their leader, The One, is described as every evil, every corruption. The nameless One is the anti-thesis of "Live free or die." The Animorphs, aboard The Rachel, are caught in an impossible situation, but instead of surrendering their selves, they pull a crazy, death-defying stunt.
The One and his capture of Ax is definitely a set up for a great sequel, much more so than the Underminer situation (which actually got
its own video game, which might say something). That's because K.A. treated her readers like students, not consumers. She anticipated that a number of her readers had also developed an interest in writing. As thematically conclusive as the ending was, it was also a fan fiction prompt.