Some other possibilities:

Happiness is only relevant once you are certain of survival.

Focusing on happiness is a sign that you have reached the end of your journey (be that an individual or the state).

Happiness is a binary state in an analogue world. A good meal eaten by a starving man is equal in happiness to a private jet purchased by a freshly minted billionaire. And the happiness lasts equally as long.

Also, you are correct, decreasing mortality is the primarily cause of population growth which is due to peak in 2050 at 11 billion before starting a gradual decline.