Why is increasing efficiency always net good?

Because we get more than we had before. As costs go down, the risk of innovation goes down, we're able to be more responsive, able to do new things. People are able to have more variety to meet their individual needs and preferences.

In short we get more or work less and the overall quality of life for everyone increases.

That's not universal. Good example here https://en.wikipedia.org/wiki/Jevons_paradox