This seems like they are missing something important. Evolution is driven by those genes that get passed on. If absence of the gene produces females and if females do not ever carry the sex gene it's basically impossible for the gene to be lost. They are over extending the prediction based on the chromosome getting smaller but to suggest the key gene will just disappear is just silly. For a genotype to become dominant in a species it needs to convey some sort of advantage. Usually it's a survival advantage as this correlates with reproductive success. But here we are strictly talking about a reproductive advantage. It's just impossible for that to become the dominant trait as it is a direct disadvantage. The gene (or lack there of) can't actually get passed on and therefore it can't become the dominant genotype. This is strictly regarding the whole "extinction" argument btw. Evolution just doesn't work that way