It's not guaranteed that Wal-Mart low-cost producer strategy will win forever, but right now and for the next several decades, it seems to be a durable advantage. Companies like Target and Kohl's are new entrants as well, but they're not taking Wal-Mart head-on, but trying to differentiate by being a little more up-market.
Eventually, Africa might take China's place as the world's source of cheap labor, but by then China will already be a First World country and no longer need exports as much. China will have better things to do.
If America keeps on continually pouring money into China at the same rate that it has been, year on year, for about the next 100 years, then China might be something.
The rate of return still won't be anything to brag on though.