Maybe it was meant to be a downer. For nine years characters, (people), have suffered and even died, while we laughed with the people who caused their demise. Though we love them, Jerry and company can be viewed as bad guys, at least from the other characters perspectives.
Perhaps it says that we are as callous, as a people, as Jerry, George, Elaine and Kramer are. That there is retribution, (even if it's a one year sentence). That you can't expect to treat people like that without paying a price.
And as the viewers, we were entertained by other people's suffering. So in a sense, we were punished by being denied a happy ending.
Maybe it's making a point about, us, the viewers
Sure it's only TV. But it makes you think...
Lou B. .