For some fluff, and out of respect to the Great Father - what if they use them hard until they die in battle, and then recycle them (back to raw metal if need be) to reuse in further mech construction?
Mind you, specifically melting down the IS and Armor (or what you will) of a Mech and sending that metal off to production facilities might not be super time/cash efficient compared to mining... but it both fits the Clan ‘efficiency with limited resources’ ethos and harkens to taking the genetic material of their successes to build future generations.