@RyoukoKonpaku Generations of inbreeding would purify (and improve) the genome and also eventually remove recessive deleterious mutations from the population. This is especially noticeable in isolated island populations like the Galapagos, where inbreeding depression has been nullified completely in many species. Inbreeding resulting in "non-viable" offspring is actually the result of haphazard breeding from an initially outbred population. Outbreeding only initially reduces the frequency at which recessive mutations are found on both copies of the gene, but over time the mutation load will actually increase to levels where a decrease in fertility compensates for the rate they appear.
This is also only tangentially related, but anyone who whines about incest in manga/anime is not just hypocritical, but selfish to boot. Despite the incest tag here being slapped on everything with the most liberal mentality possible (that includes one-sided affections, no actual sex occurring, NBR and general clinginess), this theme is one of the least common in the medium and even if it does occur, there's a good chance it gets shafted because of legal and social pressure. Yet, these people go out of their way to search for this rare theme they have no interest in and then either introduce dubious hypotheticals into the equation to denounce and vilify it when they could easily leave it to the niche audience it targets instead or leave general flame-bait in some impulsive knee-jerk reaction. Imagine if I were to go to the comment section of every single yuri/yaoi manga and started giving off about the risks of STDs, gonorrhea, syphilis, non-gonococcal urethritis and penile discharges, or I went to any manga with a romantic interest over the age of 30 or under the age of 20 and spouted off statistics referring to the similar (or greater) risks of producing birth defects as an independent metric compared to inbreeding starting from an outbred population? It would be obnoxious and ignorant, but by the same mentality these people are working off, I could very well do it and they could say nothing about it because I'm throwing their own tactics back at them, especially since all of those themes vastly outweigh incest manga and anime by an inconceivable margin.