I mean, like this is an idea, but not really one with any foundation. While the Witness may be called the First Knife, it doesn’t implicitly mean there will be a second knife. Rather, the title is meant as an assurance that the Witness was the first implement in this game between the Winnower and Gardener and that should their be another knife, of which their is no guarantee, the Witness would be the first. This isn’t like Rhulk being the First Disciple, making him arguably the greatest. Secondly, the Witness was a specific answer to the universe. If the Witness failed, what point is there in returning to using the “Knife”? No, rather the Winnowers attention has changed, and no longer is the Winnower trying to see the end of the universe as the Witness intended. The Final Shape is done and dusted, and that specific answer has been made irrelevant and obsolete. There will be no second Witness.