>>There is no reason to think AIs can't learn how to code defensively for this sort of thing.

For the exact same reason why there is absolutely no technical reason why two departments in a company can't talk to each other and exchange data, but because of <whatever> reason they haven't done that in 20 years.

The idea that farmers will just buy "AI" as a blob that is meant to do a thing and these blobs will never interact with each other because they weren't designed to(as in - John Deere really doesn't want their AI blob to talk to the AI blob made by someone else, even if there is literally no technical reason why it shouldn't be possible), seems like the most likely way things will go - it's how we've been operating for a long time and AI won't change it.