It depends on what you mean as a "political death knell". If you mean winning the presidency, yes, it is, but if the standard is lower than that, then for several people on your list it was not a death knell. Bentsen continued to serve as the Chairman of the Sen. Finance Committee before becoming Sec'y of the Treasury and Dole had a huge career in the Senate after 1976. Kemp's career was over before Dole pick him and Quayle's career was over, too. Even had the GOP won in '92, DQ was never going to be the nominee in '96. As for Lieberman, my guess is that he'll continue to have a career in the Senate as long as he wants one, could run for gov in CT, or could be appointed to a Cabinet post in a Dem WH.
Lieberman is a bit of an exception in any case, since he happened to represent a state that permitted him to run as a candidate for both the VP and U.S. Senate. If that were not the case, I think he would have disappeared as quickly as someone like Ferraro had. For him to come back four years later and have such a dismal showing in the Democratic primary campaign was all the evidence you need of that.