There was a certain inevitability in existentialism’s decline, but at least the early existentialists were serious.  I count four stages in the philosophy's devolution.

1.  One must choose between hope and despair.

Soren Kierkegaard, 1847:  “To relate oneself expectantly to the possibility of the good is to hope.  To relate oneself expectantly to the possibility of evil is to fear.  By the decision to choose hope one decides infinitely more than it seems, because it is an eternal decision.”

2.  But there is no ultimate ground for the decision.

Albert Camus, 1942:  “There is only one really serious philosophical question, and that is suicide.”

3.  So what?

Thomas Nagel, 1971:  “If sub specie aeternitatis there is no reason to believe that anything matters, then that doesn’t matter either.”

4.  Giggling into the abyss.

Jailblimp, an absurdist promotional video, 2017:  The descent of a little girl’s birthday party into bizarre and inexplicable events evokes no reaction from her except to check out her presents.