No, it doesn't assume the population has to increase. The CURRENT population is 8 billion. Almost all of those 8 billion will be dead in 100 years. In addition to the current 8 billion, millions more are born daily who won't make it to 99.5 years, and 100s of millions more will be born in the next 5 years who won't make it to 95, etc, while very few of the current 8 billion will still be alive in 101 years.