Do you think Sanford and Son could have lasted well into the 1980s?

Well, if the latter Sanford (https://web.archive.org/web/20061031125314/http://www.jumptheshark.com/s/sanford.htm) series (http://www.tvparty.com/mysfoxx2.html) (1980-81) is any inclination, then probably not.

Yeah, I've noticed usually when a sitcom cross over from one decade into another, it starts going downhill it seems... Like when the Beverly Hillbillies crossed over into the 70s

Not Really, Sanford and Son always would be better off in the 1970s more often.