Grounded in a deeply held confidence I'd wager. This is a really neat find in what is essentially in our own back yard. The old theories will continue to fall one by one. Just how far back will the bone diggers find evidence of civilization is the question. 15,000 YA to 50,000 YA is quite a jump, but if I were a betting man I'd bet they will eventually find evidence older than that. Call it a hunch. Since I don't really have anything else to base it on ;^)
FGS
I guess my blimp joke went over like a lead- uh, forget it. ;')
My view is that humans have been in the Americas for a very long time, and remains will be found pretty much as deep as anyone cares to keep digging. Problem is, the preservation of remains is rare, as shown by the vanishing of much more recent remains, like, say, my great-great-grandmother.
If I had to guess, I'd say the Barringer impact ("Meteor Crater") had negative consequences on North American inhabitants (and animal species) over a wide area, perhaps opening the door a bit for a more recent wave, or more than one. Also, glaciation dropped global sealevel, meaning that plenty of potential sites are now part of the continental shelf and submerged.
More to the point, the water's edge was lower altitude, and all other things being equal, the lower the altitude, the warmer the climate. Getting to the Americas from Europe or eastern Asia (or vice versa) by boat was probably commonplace, and probably happened more than once.
:')