The point of this thread is meant to be trying to come up with practical and applicable solutions. Which is why praying that predicated on what's cutting edge now we can predict what is likely to be commonplace in x-decades is pointless. Its also something that I would hope given the failure of such predictions over and over again since the 50'ies and that nearly every major technological innovation thats really 'changed life' has come out of the blue. I'd hope it would be in decline now as a way of thinking.
Even if 3d printing advances massively, and i stress 'massively' to the point where its affordable to use it to produce complex devices out of metal with internal moving parts. So I can press a button and out pops a ford fiesta. How does that solve any of problems we are talking about here? At best its going to massively decentralise and automate manufacturing. Its not going to be able to create more raw materials. Nor does it address any of the problems already mentioned surrounding potable water, food and energy.