Well it seems obvious to me that there should be significance (one way or another) in the judge's sentence of only one year. Remember, there was a maximum possible penalty of five years. Plus, this was emphasized in lines such as "It's only for a year." I think there are three possibilities:
1. Definite plans are in place for a continuation of Seinfeld after one year's time. This way they get all the bang of a finale (though I didn't find it to be much of a bang), Jerry gets his "rest from being funny", and NBC gets to have their very profitable sitcom, albeit one year later. But if this is true, can all of those in the know be expected to keep this a secret for an entire year? But then again, a rumour of it continuing can be met with a rumour of it not continuing, so that the uncertainty remains the same anyway.
2. Seinfeld is over forever. The "year" has significance only in that the writers wanted to tantalize the viewers with the possibility and keep them hoping, but in fact, the end is the end.
3. They incorporated a sentence of only a year, because they want to retain the _option_ of continuing the show in a year's time. Jerry might have agreed to consider continuing the show after a year's rest, but may not have committed to it. Perhaps NBC told them to incorporate it into the finale because they are hoping that the show will end up continuing. If it does continue, it makes perfect sense (i.e. they have now gotten out of jail), and if it doesn't, well most people thought it was the end anyway...
What do you think?