You asked for steelmanning. I speak as someone who's much on board with much of what you say above. 

For me, the key reason why I'm worried about consumerism is that it can easily become an addiction and it tempts me to give the unimportant things in life too much attention.

Imagine if someone asked why cocaine or alcohol have a bad image. Obviously, cocaine and alcohol make people happy -- very much so. But they're also addictive. What was supposed to be a tool for achieving happiness turns into a master. And then it destroys my ability to focus my full attention on the things that truly make us happy: relationships, God, achievements, nice meals, generosity, art, etc