I agree with you, but once there are orbital factories (and how far are we from that with companies like this? 20 years?), the cost to "launch" becomes next to nothing.
Let's target rational exuberance.
I'd be more pessimistic on the grounds that there isn't a huge market for space-based factories. Making the best parts requires heavy machinery (whether or not it's automated), which you can't affordably launch into space.
Good match for space fabrication, too. Zero-g makes it much easier for an air nozzle to dislodge debris. Feed it big ingots of metal straight from the asteroid smelters. Not sure if you could do the milling in a vacuum, but it would be worth a shot. Downside, greatly limits cooling. Upside, easier recovery/purification/recycling of shavings. A big electrostatic charge on the ingot and tools should make the shavings literally fly off of the work to a collection plate.
Unless you are trying to obliquely refer to casting, which should be pretty much identical with or without gravity/air. And still needs tooling to make the molds.
(And please don't say that the molten metal could be a feedstock for a printer. That would be a circular discussion.)
Not saying it would necessarily be cost-effective, but it's an interesting idea.
I guess you meant something more like blacksmithing. Chamber could add heat by microwave induction. No air to oxidize the surface so you might be able to work trickier metals. We already have power hammers, we already have 6DOF manipulation tables. I'm not entirely convinced that a power hammer could do the work accurately enough, might want to use a milling machine for the final fitting.
I think you're right that automated factories in space will reduce the actual launch costs (just the cost of getting your junk into space). The rest of the cost reduction has to come from reducing the cost of manufacturing. Accepting additional risk can certainly help here. That's a big change of mindset for the aerospace industry.
In my example, the cost of materials and manufacturing was a fraction of the actual launch cost. I don't think that's typically the case for traditional spacecraft, but I'm not positive about that. Adding more or heavier parts increases the mass, which requires more energy from the launch vehicle to reach orbit. I find it entirely plausible that adding redundancy increases launch costs as well as manufacturing costs, but I don't know how the big boys arrange their launch costs. (At PolySat, our satellites were so small that we paid per kilogram. Other satellites can be the size of a van or even a school bus.)
I think 20 years is a bit optimistic for space-based factories to reach the point where we can dramatically lower the costs of launching a spacecraft. I think a kilogram of aluminum mined in space and a kilogram of aluminum mined on Earth would not be fungible, and would thus have very different costs. Initially, I think the aluminum available at the space-based factory would cost significantly more than ground-based aluminum. (The company that builds the factories will have to recoup their costs, as will the companies that mine the raw materials. And at least initially, they're going to be paying to launch those spacecraft from Earth.) Eventually, we could reach a scale that the cost of materials in space is low enough to seriously discount traditional launch costs. I just think there's a lot more than 20 years of work to get there.
They're going to make a movie about this starring Bruce Willis. Throw in Ben Affleck for comic relief. They're going to call it "Armageddon" and it's going to be a blockbuster.