Basically I am referring to every hominid skeleton that has ever been found. That's what I consider bigfoot to be a primitive hominid living along side modern humans. More and more hominids are being discovered as having lived contemoraneously with modern humans.
So its accepted my mainstream scientists that archaic hominids existed concurrently with modern humans in the past. So why couldn't they today?
It may seem less likely to people today, with the destruction of wild areas and the high human population. There have been tracks and sightings for many years of the so called "bigfoot" or "sasquatch" in North America, going back hundreds of years to Native American mythology. "Wild men" exist on other continents as well in Native Folklore. Of course that doesn't necessarily prove that these wild men are "only" mythgological. Animals well known to science, such as wolves, ravens, etc. populate indigenous mythologies.
In terms of North America, it may be more likely that a small population of archaic humans would be able to survive, since most people don't spend any considerable amount of time bushwacking through dense woods, away from clearly marked trails. The number of trapping and hunting licenses purchased goes down every year and fewer and fewer people make their livings in the woods. But among people who do, you often hear accounts of bigfoot encounters.