Home production will never be as sophisticated as what can be achieved in a factory. Anything that can be done in a home can also be done in a factory, plus some.

Lets think about this though, how are you going to print a vulcanized rubber tire? How are you going to print an engine block, a car body, and a windshield?

Okay okay, lets say that in some sort of manufacturing singularity we can print all of those things... cars are still big, and this printer is going to be big as well. Where do we do this? In my garage? Well, I don't have a garage, I have a shared parking garage, but lets say I had a two car garage. One space for the printer, one space for the car. Printer prints all of my parts, I assemble (okay, perhaps I have robots (printed? themselves robot assembled?) strong enough to assemble it), fill it with fluids, and drive off into the sunset. Was all of that equipment used to print and assemble it generic? Do I have other uses for car-building sized printers and robots, or is that just wasted space in my garage now? Do I grind all that stuff down into printer food? How long does that take? How long did that stuff take to print in the first place? How long did my car take to print? Hell, where did I even get the material to build all of that stuff in the first place, and where do I put the printer food when I'm done? Do I order up a few tons of steel and have them come back a few weeks later to pick up the barrels of steel chips when I'm done?

This all seems pretty intensive just for one car, but there are some obvious improvements that can be made. For instance, instead of doing all of that work for just one car, I could print out two cars instead. Setup/demo time would be the same, so it would be more efficient. Now I don't need two cars, so maybe I could sell one of them to my non-hypothetical self who doesn't have a two car garage in the first place. Hell, we can even do better than that still; my neighbor Joey doesn't really feel like clearing out his garage (his printed model trainset is in the way), so I'll print and sell him a car too. In fact, maybe I'll print one for the whole neighborhood... maybe I'll rent out some larger space than my spare garage space to do this, get a few production lines going in parallel...

Oh shit, I just re-invented the wheel.

3d printers will take off, but they will nevertheless not represent a threat to the concept of centralized production. Only profoundly incompetent companies will be threatened. Any other company that resorts to lawyers will be overreacting.

The question is how much capital is required to start. At the moment, you need to either be a huge corporation or (like Tesla) market yourself well and get a ton of investors to start making cars.

If the price of building a small factory dropped enough, maybe your local library/school would have an open factory space where people go to print custom cars.

If you don't want a custom car, it'd probably be cheaper to buy one though (partially because the availability of such printing would force manufacturers to drop prices).