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.
A well made temporary crown can be made to last upwards of 2+ years if necessary (but the crown and tooth needs to be followed closely, examined every 3 months or so to make sure that the temporary crown is still serviceable and that there is no leakage of the cement).