Hi

I have never known rich running to kill a plug - lean obviously does.

It could be that your bearings ( or other metal parts) are dissintegrating and the shards are breaking the element of the plug - look for pit marks on the plug base etc.
I have found that if the element breaks in flight it does not nessaraly kill the motor but when you try to start it again it is dead.

Also check for air leaks that could cuase your motor to go lean in flight.