How do you know what can happen in nature? What are you basing this post on?
How about a couple billion years of evolutionary history and a whole lot of genetic research?
Plants don't pick up fish DNA.
Regardless, this isn't really the issue, however there are some legitimate concerns.
First, when a genetically modified plant is openly pollinated, the effects on the ecosystem are not well known. We have a nasty habit of doing things which seem innocuous at the time, but screw us in the butt because we didn't think of the ramifications. If a wild type plant is suddenly given the ability to out compete others species then that screws the pooch. Look up purple loostrife. It's an introduced species that's destroying our wetlands. We don't need more of that sort if thing.
Second, the robustness of a species depends upon diversity, a valuable food source with only a few variations is a recipe for disaster. Because an organism is immune to a particular disease doesn't mean that it always will, nor that a new one won't develop. In that case most of the crop in question will fail, because we're dealing with a shallow gene pool.
Third, there is the problem of patents. It's all well and good to say that a crop can produce more, however if a plant is designed so it is infertile then that requires a people to be forever dependent on a company for seed. If there is a disruption due to economics or supply, there is no carrying over from the last crop.
Fourth, all the above depends on several things. Horses don't pollinate, qualities like taste do not offer a survival advantage in the wild.
In the end it all depends on the particulars of the modification, and it's implementation.
We're far from discussing that intelligently it seems.