Eh, I'd say it's fair to give them a short window of patent protection. Say 20 years, akin to pharmaceuticals.
The only exception should be if the protected variety has a monopoly. Then it immediately loses patent protection.
Also, all patents are monopoly. That is actually what a patent provides, it provides a limited government sanctioned monopoly over the invention.
Come on. Every product is then a monopoly because it’s the only one of its kind.
If your GMO seed plants 80% of its crop in the country, no patent. Otherwise, farmers choosing to plant your seeds are making a choice and should pay.