Just for the fun of it, I am going to continue this friendly debate.
Kelly and Scotty did not by any means buck rule and regulations all the time. In fact, it was their sticking to their job and their orders which caused Kelly so much angst at times. Now and then they tried to buck orders, like when Kelly was told to kill his old friend, but at the end of the episode, there he was doing it, and righteously so, after Scotty was shot. Kelly and Scotty had freedom in the way they did their job, to some extent, but their job was to follow orders, and do what the Pentagon ordered them to. If they had not been diligent in that way they would have been out of work. The Pentagon did not glorify them, as we all know, and had a lot of other agents to work with, so Kelly and Scotty did what they had to do to get the job done, no matter the cost to their peace of mind.
Culp himself said that by his 30s, getting hurt on the job as an actor, well, HURT. By 51, when he was on TGAH, he said he did some action scene and couldn't get out of bed the next day. Plus, his one track injury in high school caused him significant arthritis in his neck. I think we have to apply this also to an aging Kelly, who has a significant history of injuries himself. I can't imagine that as he got a little older, getting hurt, getting punched etc. , having some arthritis, would start looking very UNappealing, and not something like a typical adrenaline rush. Only in the movies do punches not really hurt, and bullets not seriously wound. If we are looking at "real" life to analyze Kelly's aging, it is simply realistic to understand he would have to pull back from wanting to stay active in spying and danger, at least if he kept inhabiting Culp's body, which DEFINITELY didn't want to even act out simple fight scenes by his early 50s.
Last, I don't think folks are truly considering the importance of a child in Kelly's life. We know he had a problematic childhood with his mom dying early, which caused him to lose the special summers he had with his Aunt and Uncle. He was desperate for family in his life, I think, which is why the special closeness with Scotty caused him to see Scotty as a brother, not just a friend, or spy partner. He even called Scotty's mother "mom", himself. If Kelly had loved a woman and had a child with her, we can make all sorts of extrapolations that she would have, say, upon her early death, asked him to get out of active work entirely for the sake of being there for their child. Anyway, Kelly did have extreme loyalty and the importance of family as huge personality traits; I find it odd to believe he'd drop his son off somewhere (the astronaut to be!), and simply be dallying all over the world, still drinking, sleeping and being very egoistic. I think he would see those years as past, and I do think he could settle down into a career which was half in the field, supervising, and half shuffling some paperwork, to be around his boy. As for the paperwork issue, please recall he and Scotty did have to regularly do paperwork as field agents; writing up and turning in expense accounts, and the accounts of their recent case.
Anyway, Kelly as office dweller with a lovely relationship with his son (showing that he had a big part in his life), still seems to me like a very valid possibility of Kelly's later years.