It's sort of an imperfect analogy, though. The people who say things like that are thinking of it in the "You wouldn't download a car?" sense. You can already download 3D models from a place like Thingiverse, so if everyone had a printer, we could just print new household items rather than purchase them. It's a functional thing, not an aesthetic one.
The issue with this is that just because they make things doesn't mean they make them _from nothing_. Are you going to keep not just a printer, but large vats of materials hanging around your house? I don't really think so.
I tend to imagine the large vat of materials becoming part of the corner convenience store/gas station. "Hi, I'd like $50 of gas, $38 of aluminum composite solid #3D2, and this bag of chips."
Alternatively, it could coincide with the one-hour photo lab: "plug in your USB drive to begin uploading model." prints out receipt
You still need a volume of stuff that's larger than the thing you want to build. A convenience store sized package would only provide enough material to fab something smaller than the package itself.
Right now, you load the material into the machine with a small crane-like thing. So theoretically, I guess you could hook up piping instead. But that's still a honking big machine, and you'd need a machine that'd be larger than the things you're building with it, really.
I think eventually, it could happen. It's just not going to happen any time soon.
My favorite theoretical material supply scheme is grabbing carbon out of the air and re-configuring it into whatever you're printing with ;)
OK. And the things you make with your printer still have to compete with stuff from custom processes. So e.g. your printed tea cup will probably not as good as a normal tea cup, but it may be more convenient to just print some cups instead of going out and buying some.
Traditional manufacturing processes have really expensive tooling, but then each part is really cheap. Printed parts are always the same price. There's an inflection point where if you're going to make more than a few tens of thousands of things, it's cheaper to make the tooling. But if you're not going to make that volume, printing is cheaper.
Like many things in life, it's all about the best tool for the job.
