They act mostly as a safety net (especially at lower levels).
It is an accepted fact that due to the unpredictability of the dice, players can have unlucky streaks, and monsters can have lucky streaks. A crit from a monster at the wrong moment, could instantly drop you down to 0 hp when you thought you were fine.
While deadlier combat is the goal of this thread, I don't think we want the deaths to be completely arbitrary and out of the blue.
That's. The. Problem.
When you said that t"death saves do have a legitimate purpose." I kinda expected you to do more than list all the ways they make combat less deadly and shield combat from the possability of being at risk of death.