I'll say it again, it's supposed to be generic and no state in particular. Better put: It's every STATE. The 'four corners' in the movie were not the four corners, Homer takes Bart to the top of a hill and says you can see all the states that border Springfield, then names a State from each region of the country. This is necessary to accomplish the purpose of the show, to take a humorous look into the American culture.

Being from Illinois, I wish I could tout our Springfield as the Simpsons' Springfield, but there's just no way. SF, Illinois doesn't look anything like the show, no place does because it is fictional. It was thought up in somebody's head as a sattire of our lifestyle back in the 1980's, even though the show has held up well through changing times. Three-eyed fish??? Remember the disaster at three mile island penn? It's literary commentary. If the town was called "americaville" the show would retain the point. Choosing the most popular town name in the US does not all of a sudden pinpoint the location.