The math might be easier, but the physics is much harder. How do you 3D-print a metal object? It's an easier bet to say that computing power will continue to increase than it is to say some magical high temperature sintering process will be made for home use.
Lost wax casting from a 3D-printed mold would seem like a more straightforward process than CNC machine a brick of solid metal, honestly. People have done this; I've seen blog posts.
Sintered metals are never going to have the same properties that cast, forged, and/or welded metals have. Such parts may be very strong compared to, say, plastics, but there will always be a huge class of parts (especially for machinery) which just cannot reasonably be made that way.
But that's fine, additive manufacturing doesn't have to be the end-all be-all of manufacturing. People should concentrate more on leveraging the strengths of 3D printing as much as possible rather than trying to make it be some jake of all trades manufacturing tool that it can never be.
There's quite a few processes that allow the production of metal parts. The big advantage comes when you produce titanium parts using Selective Laser Sintering. Titanium is very hard to machine using traditional processes, and the waste produced means it's very expensive. SLS deals with both of those issues. The principal downsides right now are inconsistencies in material properties and surface finish. It will be interesting to see if we can solve those problems.
Personally, I'm super excited about the possibility of ultrasonic consolidation hybrid printers:
http://reprap.org/wiki/Ultrasonic_consolidation_Hybrid_print...
As a hybrid approach, it wouldn't be quite as flexible as full 3D printing, but still, it's pretty cool!
