You are correct about what genes exist now and there is some discussion about the “first†Americans suffering a mass extinction of some kind. However, MDNA studies have proven very useful when tracing migration of the current gene pool. Along with bone analysis as correlation there is a strong pattern of migration developing. I suspect that we will find that there were three or even four distinct migrations in ancient times to the Americas.
These studies don’t do anything but prop up existing views. One DNA sequence that was found common among sampled folks with PreColumbian American ancestry was found in just one sampled person in Asia. The conclusion drawn? That that one person in Asia carried a DNA sequence that had otherwise died out in Asia, but had been carried into the Americas by someone in the small, Beringia-walker population; obviously, A) this should show that reliance on DNA doesn’t make sense, because the arguments are constructed to justify the results (the DNA sequence isn’t found there in great numbers now, therefore it must always have been uncommon, or died out for mysterious reasons, or just hasn’t been found because so few folks have been sampled worldwide), and B) that the Bering migration route didn’t have a one-way sign, or a limited time frame.