People can change a lot. For example, the hints of the Crockett that we saw in Vietnam were very different from what he was like during the show. He had a very distinct arc during Miami Vice. It ended with "Free Fall," but since he's still alive when the credits roll, he's still got more story to tell. It's just a very different story.
So, I could see his becoming a Nash down the line, after he'd taken a break, gone fishing for a bit, and gotten his head straight again. A lot of blood had flowed under Nash's bridges in the past (sorry, couldn't resist), and under his cheerfully sardonic exterior was a fair bit of darkness and cynicism. He at least had a Crockett-like past.
I think Stan's killing those guys who framed him got him off the hook. More or less. Not sure he still had a career with law enforcement, either, afterward, or whether he became someone like that guy in "Lend Me An Ear."
With Tubbs, it's harder to say. Maybe he became a private detective. I could see that.
The Historical Meow
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Nash Bridges was a great show. He was just like Sonny Crockett. Then again he's was nothing like him.

Well, it was '89 and a lot was about to change. Post-Reagan, much of that drug corruption in the DEA, etc. got cleaned up, and the kind of bozos who got Crockett and Tubbs bounced were about to get bounced, too. Law enforcement also started dealing with job-related PTSD much better.
Thing is, I'm not sure Crockett and Tubbs would have wanted to be cops again, since they thrived in that Wild West atmosphere and didn't like a lot of red tape. But some fans of both shows have noted that Nash Bridges could easily be an older, wiser version of Crockett.
Innsmouth Free Press
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