[pdonis]

> the R^2 for all three fits was above 0.9, suggesting the exponential growth model is reasonable

What is the R^2 for other fits? Say linear or quadratic (the most obvious alternative choices)?

> it's non-sensical when you look further back in time

Of course, because the factors that are at work in producing the data are not constant in time. So there is no reason to expect a single curve fit with a single set of parameters to be applicable for all times.