Linus Torvalds, for example, didn't actually try to write Linux from scratch. Instead, he started by reusing code and ideas from Minix, a tiny Unix-like operating system for PC clones. Eventually all the Minix code went away or was completely rewritten -- but while it was there, it provided scaffolding for the infant that would eventually become Linux.
The creation of Linux is, in part, the stuff of legend. But ESR does make an interesting observation on one role that Open Source software can play in building complex systems. Rather than viewing Open Source software as a permanent fixture of a larger system, why not view it as temporary scaffolding to be replaced, in part or in full, after the system has been fully implemented and all of its requirements are known?