And how, pray tell, will the rentiers ...
Contract manufacturers are not "rentiers".
make even more money if noone can afford to buy what they make because they're unemployed?
You should read up on Comparative Advantage. Here is a simple example: Annie makes apple pies. Bill makes baskets. Each day Annie makes two pies, and Bill makes two baskets. Then they trade one pie for one basket, so they each have one of each. Then Mike, a manufacturer comes along. He can make ten pies as cheaply as Annie makes one. So Bill trades a basket to Mike for ten pies. So is Annie unemployed? Of course not. She switches to making baskets. The result is that now, each day, both Bill and Annie get one basket and ten pies. They are both better off.
Ah! But then Mike starts manufacturing baskets too! At a tenth the cost that Bill can make them. Now Bill and Annie will both be unemployed, right? Wrong. A pie is still worth one basket. So Annie can go back to making two pies a day, and trade one for a pie, and Bill can make two baskets a day and trade one for a pie. They are no worse off then at the beginning.
So automation may cause short term unemployment, as workers retrain, but it should not cause long term unemployment. If automation is comparatively better than humans at some tasks (as it certainly will be), people will be better off. If it improves productivity evenly, then people will be no worse off. This is not just theory, but also corresponds to reality. Economies with rising productivity do NOT have mass unemployment and poverty. Productivity improvements result in rising living standards. In fact, they are the ONLY thing that can raise living standards.
What is happening at Foxconn is a predictable result of China's economy maturing. The service sector is expanding. Manufacturing will also expand, but manufacturing employment will fall. Internal demand will rise, and exports will be less important. This is the same thing that happened a generation ago in developed countries.