There is only one threat that could wipe out bigfoot as a species. Disease.
Contentions that they are being forced into extinction by habitat loss are not based on either evidence or logic.
A spotted owl lives in a very specific habitat niche. Damage that niche, you damage the species.
Bigfoot, however, are immensely adaptable. They've been reported in every major terrain, and reported to take advantage of a wide range of food sources. They can also apply intelligence to adapt to changes as neceessary.
Other, less intelligent species, both predator and prey, that are adaptive are bounding back and spreading into areas where they had once been hunted out.
I found, but have not been able to locate since, an oral history from a Southeastern Native American tribe that stated that bigfoot had once been numerous, but that when smallpox and other European diseases were introduced back in the 1500's the bigfoot population was hit even harder than the Native American population. So hard that for generations the surviving Native Americans believed that the bigfoot had completely died out.
If this were the case, it might take centuries for their population to rebuild. It could also result in isolated regional pockets, which could account well for the regional variations in both physical size and behavior.
It may be that they are just now, under the same conditions that allow other adaptive species to thrive, once again achieving larger populations.
If so, this will work against them, as internal population pressure drives them to expand into more areas, and inevitably into more frequent contact with humans.
I believe that there are more of them than most people think, and that their numbers are expanding at an accelerating pace decade by decade.
I also believe that they can and will go anywhere they want.
I also believe that they will need to occupy more and more habitat as their population expands.
They're not being threatened into extinction, they being threatened by their own success and population expansion.
Because this is what will likely result in their "discovery".