My personal belief is that MOST of the time, open source will still win out - but that isn't some sort of fundamental rule of the universe. Any discussion of TCO that doesn't bring up support is missing the mark.
In this particular example I agree 7zip is a better choice, but many other open source projects either have a paid support plan or lackluster support (meaning you'll be supporting it yourself). Both of those scenarios need to be calculated into the TCO.
One thing I heard quite a bit in the previous decade plus was that most corporate copying of non-free software wasn't due to budget but friction, of the sort the author details plus what most corporations shackle themselves with.
In this particular example I agree 7zip is a better choice, but many other open source projects either have a paid support plan or lackluster support (meaning you'll be supporting it yourself). Both of those scenarios need to be calculated into the TCO.