@Josef K those are all valid points, I'd like to add a couple of things though. Since there are GMOs for hundreds of crops, crop rotation is still possible and can eliminate the danger of soil depletion that places like Argentina potentially face. Since most farmers don't reuse seeds and buy them each season (which is one reason why the Monsanto contract worries are pointless), they could buy other crops to continue this practice. I honestly have no idea how knowledgeable the farmers are about things like soil depletion down there, but I think that if they are progressing as dangerously as you say they are, there should definitely be some measures to educate them on the subject. So basically it's not so much GMOs that are the harm, but the practices of the farmers or the corporations (who should be informing their clients of these risks).
Anyway, I came into this thread to share this with you guys:
Biological Safety Lock To Prevent GMOs From Spreading Outside Controlled Environments.