Additional note: Not only does inbreeding not always produce bad results, it often produces superior results. That’s why animal breeders do it all the time.
It may produce some desired traits but it also increases the incidence of defects. That is why you have breed specific maladies. No, there is ample evidence of what happens when humans have incestuous children even over a few generations. And there is no evidence other than speculation that Human DNA was ever perfect.