I imagine future societies will have clean drugs to enable safe perpetual happiness

I was going to argue this point, but you solved later in the sentence: " and existential depression is the main cause of death."

The easiest way to resolve this inconsistency is that there are multiple kinds of happiness. I like this description: http://sensophy.com/the-holy-trinity-of-happiness/ Happinesses are Pleasant, Good and/or Meaningful. Your imagined future wouldn't contain anything meaningful.