I do not believe that the half-life of legal knowledge possessed by in-house counsel is all that short, indeed if the notion of steady brain drain has any reality. Nothing erodes the value of what a lawyer knows along the lines of “you lose half the value of what you know every five years …”
The vast bulk of the law remains quite stable, whereas change takes place primarily at the borders, or occasionally with tectonic shifts from either the passage of a major law or a Supreme Court decision (See my post of Jan. 24, 2006: is the notion of knowledge half-life valid?). Grey matter doesn’t steadily and mathematically black out.