Boiling the frog has much less risk of causing an economic shockwave. As horrible as Walmart is, it would not be good if they died overnight because their business strategy was no longer viable. It gives US businesses that rely on China a slowly increasing pressure to find new manufacturers or even start making things here. Things in business take a lot of time and planning and can't handle rapid change well, else you get those sudden collapses. Can't build a factory and staff it overnight.