I know that Lisp has been largely stable, but are you sure it hasn't changed much? Programming paradigms advance, we didn't even have unit tests or continuous integration 20 years ago (at least not widely adopted in the industry). I find it hard to believe that even a Lisp programmer would be completely "static" across 20 years, i.e. the code written 20 years ago would be truly similar to the code he writes now.
So I'm not sure I buy this argument, unless someone has solid counter arguments.