Regarding the claim that the Bigfoot population has increased since the 1950s, or within any time frame for that matter, I don't think we have enough quantifiable analysis of reliable data to make any such assertion without a great deal of hesitation.
And so it is with a great deal of (understated) hesitation that I report that my own preliminary work in this area suggests that the Bigfoot population is not growing and is in fact under threat. While undertaking a preliminary analysis (following the analytical method of Glickman) of Bigfoot sightings at the county level, I plotted the probable Bigfoot populations on a map, and immediately discovered evidence of population fragmentation--namely, areas that must have been contiguous at some point in the past, but no longer are.
Fragmentation is, of course, a well understood threat to any animal species, and a loss of habitat results almost universally in a decrease in population. If subsequent analysis bears out the results of my preliminary work, then I can say that the Bigfoot population is certainly not what it used to be when the populations were geographically contiguous. When this decline happened, and if it continues at the present day, is another question.
I have some other Bigfoot-related work to finish up before I move on to something that could produce more definitive results on this question. Hopefully I can get to it within the coming year.