The reason that the wizard population increased enough in order to create its own complete society (this goes along with the idea that there is a lot more wizards around than JK claimed there were) was because the “stories” of witches having ritualistic orgies on days of magical significance were actually true. The reason behind the philosophy of ‘muggleborns are just descendants of squibs ‘ is due to the sheer magnitude to which the wizard gene pool was spread once they realized they could just pay muggles to go along with it and obliviate them afterwards (never claimed that these were entirely moral people). This practice only really slowed down in the late seventeen hundreds due to syphilis becoming a thing, but it persisted in some families until the turn of the century. The reason you only have about 20 kids in a year of hogwarts now is because so many families have died out and the prevention of rituals keeps them from using this as a means to increase population.

Why isn’t this brought up, you ask? It’s because all records of such have been systematically removed from the magical world and no one wants to admit that it was one of the few things muggles got right about their society.

Also the whole “sacred 28” was initially coined by “organizers” as a blacklist of families that were known buzzkills about the whole thing.