Well, first, to clarify my earlier clarification, when I referred to Kelly as a tennis pro, I meant he was a professional tennis player, who (as was the case in Woody Allen's 'Match Point'), became too old for the circuit and having no other fallback, ends up teaching tennis. Obviously the world only knew of Kelly as a tennis player and once his spying days were behind him, his cover was all he had left to survive.
Secondly, what made the 'johnson' scene so good (at least in my opinion), is that it had nothing to do with 'johnsons' so much as it had to do with two viral men of the world, reluctantly conceding to one another, in a competitive, slightly defensive way, that they were getting old. The scene was touching because it so beautifully explored the relationship that had survived all this adventure and finally, these two old friends were aging, though they couldn't quite admit it.
Regarding the reality of the script actually getting off the ground without Sheldon, well, that was what finally made it impossible for the movie to get made. Bob wanted to take the show to a deeper level in the feature film, but once Sheldon got wind of it, he pretty much put the kibosh on the whole thing. Bob's intention was to make the film edgier for a movie audience. The twin brother angle was a holdover from a script Bob wrote called 'Hooker Doyle', which eventually evolved into a project for him and Bill called 'Doyle and Doyle', but that storyline was finally scrapped.
Bob had an office full of film scripts. None of them were ever written as artistic exercises, they were always designed to be made.
I also think timing was everything and this was a time when very few TV shows were being developed as feature films. In fact, other than 'Star Trek', I don't think any big screen adaptations of TV shows at that time had been done (feel free to correct me if I am wrong).