As I was listening to Jack's last radio season (1954-55), I was impressed with how funny the show was even as Jack and his writers were clearly focusing their efforts on the television show and much of the season was composed of recycled bits from earlier years. The last season must have been a particular strain for all concerned with the increasing demands of the television program (broadcast every other week) and the virtual absences of Mary and Bob Crosby. Even Frank Nelson appeared less often. The radio show's continued strength in the face of all that is clearly a testament to Jack's greatness as a comedian. Jack could have read the telephone book and he'd be funnier than most every other comedian of his day or ours.
Some have written that Jack never did a proper closing episode a la Seinfeld, MASH, etc. I noticed that, in the final episode, Jack talked about returning in the fall. But it seemed to me that Jack knew by at least mid-season that 1954-55 would be his last. There's an episode where Professor LeBlanc informs Jack this will be his final lesson. Another has Ed the guard coming up from the vault and then returning to its safety after being overwhelmed by the modern world. Both of these had the flavor of farewell appearances, which, in fact, they were. And the close of the final show has Rochester and Jack repeating "GOODBYE" to each other, supposedly because of the soundman forgetting his cue, but clearly, at least to me, as Jack's signoff to 23 years on the radio. I actually found that conclusion to be kind of haunting.