Our family has listened to Car Talk for decades and PHC as well. The grandsons (third generation of listeners) enjoy them, too. You are right: there is a time to take the shingle down.
The Magliozzi Brothers will be missed. But it is time for a new generation of redio personalities to take over with new and fresh ideas.
Certainly we couldn't expect the tappet brothers to go on forever, but Car Talk signing off for the last time will be a truly sad event. Same with Prairie Home. I was privileged to meet Garrison Keillor some years ago and Click & Clack many years before that. All true gentlemen and significant public assets.
Statistician "Marge Inoferror"