Human labor is quickly becoming the equivalent of the horse in a world full of tractors.

Maybe so, but I'd venture to say horses today are better taken care of than they were when their use was utilitarian rather than recreational. In that way, maybe they're more valued. I for one, would like human labor to be more valued.

Of course, there are also way less horses today than when they were used for transportation. Their breeding was controlled so when there was less demand for them as transportation, we simply bred less of them. Following your metaphor, society will need to figure out what it wants to do with less required human labor. In the past, there has been a mixture of more recreation time (adding weekends & holidays), and new industries to soak up the extra human labor.

There's a discussion on the reduction of labor required per ton-mile on the BNSF railroad in one of the Annual Reports. The amount of labor it used to eat up is staggering. There's also similar productivity gains in the history of harvesting. The History Channel's Modern Marvels show has an excellent episode on it. The metric there is how much labor per acre is required for harvesting. From using hands, to a sickle, to a scythe, all the away to modern combines.

When teachers are asking my elementary and middle school kids what careers they want when they graduate, I like to remind them that people make careers out of being on YouTube, editing videos, and adding it to things they already do, and that's something that wasn't invented when I graduated.