Later shows like Homicide" (1993-1999), NYPD Blue (1993-2005), and Law and Order (1990- ) are still maintaining popularity through the same basic premise as shows of the 1970's: dealing with relationships with returning characters.
There are many more detective shows (No, I haven’t forgotten "Murder, She Wrote"!) but the ones mentioned give you a good idea of how the genre has changed through the years. My favorite? That’s easy, James Garner in "The Rockford Files". Other than the terrible clothes styles and cars of the 1970's, that show still holds up as strong as ever today. (Comments? email@example.com