they will still spread. farmers poison the crap out of p-dogs. we shoot them by the hundreds and yet they still survive. granted they live underground, pigs aren't stupid animals. they have managed to spread, and survive quite well. just look at all the states with pig problems.
not all farmers or landowners are going to care if the hogs are on their property and even if they do care, less of them are going to be able to control them, or kill them all off. I don't think it will be an overnight invasion but I believe they will still spread.