But for me the worse is the fact that they sell seeds such that the next generation is not fertile (will not grow). So you cannot just plant some of last year's crop, as farmers have done for millennia.
You're right that the technology to make such seeds has been developed (and patented). However no company is now, nor have they in the past, sold seeds genetically engineered to produce sterile offspring ("suicide seeds" "terminator technology" etc) and this is one of the most frustrating pieces of zombie misinformation to confront over and over again in the debate over GM crops.
There are patented seeds where you are not permitted to resow the seeds the next year (once more because of patents), but regardless of whether or not you think gene patents are a good idea (I do not), in the event of a series economic/social/natural disruption, farmers would just plant the seeds anyway and ignore the intellectual property laws.
I agree with you that selling plants that are designed to be sterile is indefensible on both pragmatic and ethical levels.
For example, and yes, this is real, they make crops that have weaknesses so that you need to buy more pesticides of the kind they sell.
Citation needed. I know there are GM crops resistant to certain herbicides, but in the absence of those herbicides they grow identically to their unmodified siblings. I don't even know how an effect like the one you describe could be produced. But if you can back it up I will certainly look into it.
In any formula, constants (especially those obtained from handbooks) are to be treated as variables.