There is a problem of equating being "up to date" with "competent for the position". Up to date might mean you know how to work in the syntax & convention of the stack of interest, but that's largely independent of the level of competence for overall software development & discipline. If an older programmer is solid on the latter, getting up to date really isn't a large amount of time. It's harder to assess, just like its hard to assess in interviews even if the engineer is "up to date" - look at the perennial debate on how best to interview candidates.