After you replace the laborer, what's going to happen to his standard of living ?
You're assuming an oversimplified, ideal world, where if there were no robots manufacturing in China, everything would continue as it has been.
That's not what's going to happen. As China has developed, wages have increased. They now have a burgeoning middle class. But the higher wages means their factories powered by manual labor are no longer cost-competitive with other third world countries. Already, a good chunk of manufacturing is being shifted to places like Vietnam.
This is pretty much the same situation that U.S. manufacturing faced in the 1970s and 1980s. Automakers tried to make manufacturing more efficient by bringing in automation. Unions rebelled and forced automakers to stick with mostly manual assembly. That made it a lot more expensive to manufacture in the U.S., which caused assembly line jobs to move to places like Mexico, Korea, and China.
i.e. It wasn't automation which threatened those union jobs. It was economic uncompetitiveness. Manual assembly of cars under the prevailing wages in a developed nation is too expensive compared to the alternatives - be it automation or off-shoring and shipping the finished product back to the U.S. At that point the population has to adapt or die. Either shift its workforce to higher-level jobs while automated robots take care of the menial grunt work, or watch all those menial jobs leave the country to places where the prevailing wages are a lot lower.
Under pressure from the unions in the 1980s, the U.S. chose the latter. And what followed was a massive exodus of manufacturing to countries where labor was a lot cheaper (which BTW is how the market eliminates areas with lower wages - it moves jobs to them causing wages there to rise). China is finding itself in the same situation as the U.S. in the 1970s and 1980s, and the owner of Foxconn at least has read up on his history. He's not going to let Vietnam do to China what China did to the U.S. He's going the automation route to keep his manufacturing factories in China. At least that way some jobs will remain instead of the entire factory being shuttered and replaced with a one in Vietnam.
Yes menial workers will lose their assembly line jobs. But (most) people can be retrained for higher-level jobs which pay more, monitoring and repairing the robots which do the assembly. And frankly, I find your assumption - that an assembly line worker is mentally incapable of doing nothing more than connecting part A to part B over and over all day so they will be unable to find a new job if factories automate - to be quite insulting. The vast majority of people are capable of much more than that. It's just that until technology advanced to the point where you could have machines do the grunt work assembly, there was a lot more demand for these low-end manual assembly jobs than the higher level jobs.