you are so correct - i collect floor ashtrays from the Deco years - they range from brass to c*rap pot metal of whatever was left over went into the crucible. .. did silver smithing for a bit and know metal weakens and fails.  BUT, you put your finger on the key issue, and that is maintenance, regardless of plane, car, boat.
 i imagine they machine replacements parts for older planes, also know the military and commercial aviation  has raided the "bone yard" that is in the mojave for years - not sure if they still do. 
 anymore i am scared to death to fly which i must sometimes- unfounded, i know - i've avoided certain airplanes if i had a reasonable choice, and ask about the age of the plane which always draws complete silence for  a bit. i say a prayer that the mechanics were honest and not just signing off on things, and were well trained and kept that way...  my life is in their hands - maybe more so than the flight crew.  planes are so unforgiving. oops = death too often. boats :(