Thanks for the well-argued piece. There’s a lot of fear-mongering, hysteria, and general scientific illiteracy on the subject.
I will, however (you saw a “but” coming didn’t ya? ;)), point out that there is a growing number of open-source GM varietals. There were a couple of mentions of it but it’s worth repeating. https://www.technologyreview.com/s/539746/as-patents-expire-farmers-plant-generic-gmos/
And it isn’t just patents expiring that are behind open-source. There are legitimately well-meaning researchers and organizations (e.g. Golden Rice above) that are giving their GM seeds away, no licensing fees required. Same as some people in the heirloom community are doing. I would bet that we will see more of this in the future.
Furthermore, not all patents or restrictive licenses are held by companies. Many are held by universities (e.g. papaya: U. of Hawaii); and while not GM (yet), UC Davis is very strict with licensing cultivars of strawberries:
“… strawberries are a phenomenally valuable crop, these varieties earned the University of California $50 million in royalties from 2004 to 2013.” http://www.npr.org/sections/thesalt/2014/07/02/327355935/big-bucks-from-strawberry-genes-lead-to-conflict-at-uc-davis