Question: Assuming that the film didn't underperform, would there have been a sequel?
Answer: It's hard to say for certain, but any movie that ends up making a lot of money is pretty much guaranteed nowadays to get a sequel, as it signals to the studio and producers that they now have a recognizable and lucrative IP.
Answer: A sequel is still possible, but PIXAR and Disney say there are no current plans for one. They are open to the possibility but claim to be moving away from a sequel-heavy slate to instead focus on original projects. Although it received good reviews, Onward under-performed at the box office, though that was partially due to the COVID pandemic. A sequel is doubtful, or, if there is one, it could be a straight-to-video DVD.
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