That doesn't *have* to necessarily be true.
It could make sense that insects would evolve out of this oleic acid immunity behavior, but at what cost? Would ants no longer carry their dead? Would roaches no longer avoid disease stricken roaches? Etc...
In a way though, you've contradicted yourself. To summarize, you said through natural selection, insects will evolve around avoiding oleic acids because their survival depends on it. Your argument afterward is that diseases that will spread on a massive scale as a result which I'm thinking would have a far worse result.
How would natural selection be able to allow this immunity of "stinky" oleic acid if it results in the death of those insects that require it? Or more broadly, why would evolution develop a particular trait which allows the thriving success of a species at the nearly imminent cost of its survival? I'm sure there are examples, but mathematically speaking, it would be like sacrificing your Bishop to take a Pawn. *does not compute*
Sure, not being able to feed on our crops will further limit the size of insect populations, but the species will survive. I'm sure there would be exceptions and those exceptions will be met with another form of insecticide or repellent and so on.. as has been done for many centuries.
So, in short, I'm sure the absolute *survival* of many insect species does not *depend* on our crops. It's a big planet and not insects need to stick to a strict diet of corn, soybeans etc... Sure, the population may have to be reduced, but so is the "S" curve of life.