I think that was just an "arbitrary" constraint of the problem, inspired by the practical constraint of using it to cut an onion?

Hey now, even the practical application of cutting a typical 10 layer onion was left as en excercise for the reader. Quoth:

"So, the best depth for an onion with ten layers would be somewhere between 0 and 0.5573066. I have not investigated this in depth, but this seems like a fun next step."

You are suggesting something even more advanced. :)

Another onion enthusiast has sent me python code that considers finite-layer onions, and that code will be featured in the upcoming journal article.

The two cases this solution generalizes are the vertical and radial cut method, which both aim towards a single point (you can think of the vertical method as aiming to a point infinitely far beneath the center of the onion). There may be other more optimal ways (cutting each layer individually for example), but they are not conducive to an ultimately simple strategy.