The usual reductio ad absurdum is: In another world, you are allowed to sell yourself into slavery. In this one, you are not. How can the world containing slaves be the one that's more free?
In other words, the code is free until it is added to, becomes proprietary and - if a network effect adheres - then that proprietary version can dominate and in effect extinguish the public domain version. Certainly extinguish nearly all its value. The once-again-enslaved code lives and thrives, the freeman code perishes, or nearly so. That's a good argument, but the whole problem is created by an IP system that perverts the concept behind copyright. Paintings don't prevent other paintings from competing, code with a network effect or lock in does. Copyright should either never have been applied to code, or be for a very short period. Let's say, four years or eight years. Then BSDL code going proprietary isn't a threat, it's a bonus: more free code soon enough.
