We'd need to find a way to continually power the robots.

Mass solar power, maybe?

Or perhaps, if everyone were perpetually swept away into the wonderland of the perfect happiness drug, the human body itself could be appropriated as a battery, leading to a deliberately created Matrix. </humor>

:)

The thing is, if "being happy" is the only goal, then the rest of it is just an efficiency question.

I think people view robots as either empowering us or somehow getting into lethal combat with us as the dominant life form on the planet. I seriously doubt it will be either. They'll just give us what we want. The race will naturally die out within a few generations.