The reason that large successful projects like Linux are so capable is not because it has a price tag of zero (and it often does not), but because of the feedback loop created by the viral-nature of the software license.
The vast majority of Linux is not a volunteer project -- but software developed by commercial software engineers who are being paid by a company to write software. Before copyleft, the idea that they would voluntarily share source code was laughable. The only reason they do is because they are legally required to do so.
This viral nature of copyleft creates a positive feedback loop:
1. Company uses software because it is free and solves a problem
2. they need a modification so they make it
3. they contribute back to the project because it is required by the copyright license
4. the project becomes more valuable at solving more problems that other companies have
5. Go to step 1
Breaking this feedback loop would put companies back to their natural state of not sharing. The result is that the software landscape would start to look a lot like the 80s and 90s again.
Without copyright, copyleft would not exist. And without copyleft, Linux would have been a hobby OS that died out in the early 90s. We'd be using things like Windows Server, Unix, etc. And to protect their business in the absence of copyright, they'd have heavy DRM schemes, obfuscation, cryptographic licensing, etc.
