Rigga, you are right, to a point. It is skills that are easily replaced having been taught by the experienced who now depart.
Skills are what you need at the moment but experience is what you need when your skills are inadequate, even experience to recognise your skills are inadequate.
They say that those who ignore history are condemned to make the mistakes of their forebears. Knowledge of this history is what those with experience can take with them.
Away from engineering, how many people realise just how long the PAYD issue has been going? PAYD was introduced after trials about 36 years ago! That is as far back as most long serving Servicemen will have served.