He's not making an assumption about cost == money at all. In fact, he says "...when an economist considers price, they consider the total price, including some

He's not making an assumption about cost == money at all. In fact, he says "...when an economist considers price, they consider the total price, including some intangible things like the time it takes to set up, reeducate everyone, and convert existing processes". So when he's arguing against the "claim that it's OK for every Linux kernel revision to make all existing drivers obsolete, because the cost of rewriting all those existing drivers is zero", he's saying "there is a cost, it's just not monetary." The alternative to requiring people to rewrite device drivers is having some people make the kernel backwards-compatible with old drivers. Presumably that's simpler than rewriting every driver. What else could you add to Linux if that many developers could do something that wasn't rewrite working code?