Not to get too serious too quickly, but I'm not sure why the idea of (so-called) Bigfoot falls under conspiracy theory.

Long to short, homo sapiens co-existed with other human-esque species. The idea that primitive HSs were somehow able to entirely extermination these other species feels like a pretty high expectation to me. Any survivors - and certainly there would be some - would have been able to do so because the ability to hide, run, not be found, etc.

Today's Bigfoots could be offspring of the survivers. Unlikely. But certainly, in theory, possible.