Immortality would change the annual rate of world population growth from 130 million - 50 million = 80 million to 130 million - 0 million = 130 million. If you're not bothered by the first, you shouldn't be bothered by the second.

I think the graphic at the beginning of "Torchwood: Miracle Day" represents a common belief that births and deaths are in a fine balance such that, if people stop dying, the population rate will suddenly zoom up by a multiple of the current rate.