And that is only the case with careless inbreeding, since it is just genetics and not magic. From a eugenics standpoint inbreeding is useful and beneficial as closer kin are more likely to share recessive traits, both positive and negative. And while it is more likely for family members to both have singular copies of negative recessive alleles, they will have a smaller total number of these recessive alleles as they are pulling from a smaller shared pool. With modern medicine, such as pre-screening, genetic sequencing and artificial insemination, these recessive traits can be identified in advance and selected against. Since there are less total that need to be tested, identified and prevented it actually works better with family, resulting in the next generation becoming more healthy rather than less and making future inbreeding more beneficial than outbreeding.
This is seen in nature too via what is known as a Genetic Purge. When a certain recessive trait is so deleterious as to be deadly it quickly removes itself from the population via inbreeding and the population as what some would expect. Certain species (mostly plants) actually go through cycles of inbreeding and outbreeding in order to purge build ups of negative traits. By treating negative but not deadly genetic traits in humans as something that is tested and prevented it results in an artificial genetic purge as well. >>34262979
They probably never will.