A little reality that I read last week....
Take 50 women and 50 men and put them on a deserted island. Go back in a 100 years and there will be a thriving population.
Take 50 transwomen and 50 men and put them on a deserted island. Go back in a 100 years and there will be 100 male skeletons.
It's a hypothetical that will never happen, though, so sans context that would clarify its purpose, I suspect the person who originally came up with it might have just wanted to incite anti-trans feelings in their target audience.
Besides, many cisgender people are infertile due to natural causes, health issues, voluntary surgery, or old age. Fertility is only one aspect of our sexuality, after all (and some fertile people also opt to be childless anyway).