Beautifully nailed, Lagboltz! I’ve analyzed the general data presented in the article, and had intended to 2 present both analyses here. The first analysis is based on Actual Year In Which Each Billion-Increase Milestone Was Reached vs. Rate of Population Growth To Reach That Milestone, and the second based on Actual Year In Which Each Billion-Increase Milestone Was Reached vs. Actual Population In That Milestone Year.
Believe it or not, the r2 Coefficient of Determination on the first analysis isn’t terrible at 75.4%. However, in using the regression variable and constant from that analysis, the projection of Number of Years to Reach the 8-Billion Population Milestone was -38.7. You were correct in saying that the entire world population could fit into a nursing home shortly thereafter.... a Very Small nursing home. 
In the second analysis, the results were much more believable, recognizing that few if any regression-analysis formulae yield perfection. The r2 was very good at 95.5%. The regression formula projected 2012 population at 6.6 Billion versus the Actual of 7 Billion. The projected year in which we’d reach 8 Billion is 2035, or approximately 22.7 years after the 7-Billion Milestone was reached in 2012. The regression projected a doubling of World Population to 14 Billion in the year 2135. I think it’s logical to assume, however, that World Population will slow significantly, and all else being equal, the World Population will be well above 8 Billion in 2135, but not as great as 14 Billion. Still, it’s not a reassuring picture.
There are three variables that come to mind immediately that will effect projections of analytical data. The 1st is (D) the probably-lower birth-rate per-mother, implying a lower rate of new births than in past data. The 2nd is (E1) the hopefully-improved medical technologies and drugs, implying a reduced rate of deaths than in past data. The third is (E2) A potentially reduced rate of War-related deaths, implying a lower death rate than in the past data.
The data used for this analysis includes every War-related death since 1927*, including the Millions killed in WW2, the Millions killed in Stalin and Mao’s purges after WW2, and those millions killed in various conflicts, wars, and ethnic cleansings since WW2.
*The data in the article Hobo posted would have allowed analysis to begin in 1804. Because of the significant advancements in medicine, drugs, war technologies, etc. between 1804-1927, I began the analysis in 1927; i.e., the second oldest year in which a Billion-Population Milestone was achieved.
