Unless your 3-D printer is coming with 3-d assmebly China isn't going anywhere. And why would we replace low-grade manufacturing with 3-d printing? I can't see how a "printing" process would be cheaper than simple excrusion methods used in mass today.
I just don't think you've got the vision thing on this yet. Yes, today, in 2013, for most people it is much easier to lump in their purchases into their trip to Wal-Mart and buy the cheapest variant of whatever they need that will work. That's "China" - right now. However, as the cost of the printers and material goes down, everything you acquire will be customized, suitable, and made in your living room. People will become their own assemblers because it will become second nature to them. It cuts down on time, money, effort - plus comes with the luxury of you don't need to print it until you need it. Why? Because it's not at the store or at an e-tailer, you already "have it", so to speak, when you bought the printer and source material - it's just not realized until you press the button.
Don't worry, the moment of discovery is coming for many people.