New discoveries have found exceptions to Mendel's basic laws, but none of Mendel's things have been proven to be flat-out wrong. Hail to the Father of Genetics ! The name is ma-ma-ma-ma-ma-Mendel.
Dental crowns should last on average from 10 to 20 years. Crowns are still subject to fracture and cavities, so it is important to take extra care in brushing and flossing around crowned teeth to prevent them from needing replacement too often.
Custom fitted mouthguards are expected to last 2-3 years with normal use. Chewing on the mouthguard dramatically reduces the expected longevity. Boil and bite mouthguards should last one season, possibly longer. Stock mouthguards could last several months.