"There is a persistent voice in my head that tells me that human labor is a surplus commodity in the long run."

No-- the more affluent a people become, the more goods and services they demand, in the long run, job creation always outweighs job destruction.

There are local surpluses of human labor in the run due to economic imbalances bought about by polictial differences. For instance, communism in China has artifically supress consumerism, means that instead of buying shit themselves, they sell it all to the US. The US can, in normal times, hold unemployment to very low levels and still consume loabor resources from around the globe.

The period of economic downturn will end, and worldwide job growth will continue, even as more tedious jobs are eliminated via automation to be replaced with more fulfilling ones.