Then about 17,000 years ago world climate changed. This presaged the meltdown that started about 14,000 years ago ~ but mostly it was just a change in ocean currents and major winds that allowed for a repeatable delineation of Winter Sea Ice in the North Atlantic on a seasonal basis.
What that did is create a semi-permanent population of fish and seals along a nearly stable ice mass that spread between Spain and North America. At that point people who delighted in seal hunting could make their way across the Atlantic in winter.
So, did they?
The situation in the Northwest was different. There the idea was that the ocean level dropped during the Ice Age so people had a full 100,000 years available to just walk to the Americas. Now why they should do that walking only at the END of the major glaciation, right when the ocean was rising and flooding Beringia is a good question.
Boats solve the problem. People could work their way around the litoral of the Pacific, and wouldn't need to cross through Beringia.
At the same time recent artchaeology suggests populations stopped in Alaska and the Aleutians for HUNDREDS OF YEARS and some of them actually returned to Asia ~ possibly with Japonica Rice!
I suspect humans simply could not for long penetrate inland into North America as long as the dire wolf and sabre toothed tiger were around. They were TOP TOP predators and given little provocation they would have attacked and eaten any humans.
As the Ice Age ended and the great herds moved further North for new grass ~ away from the advancing forests in the South ~ the tigers and wolves would have gone with them making it possible for humans to move in. It's possible people had been attempting to settle in North America for tens of thousands of years but were simply successfully exterminated by the predators. We should expect to find some of their remains every now and then.
“I suspect humans simply could not for long penetrate inland into North America as long as the dire wolf and sabre toothed tiger were around. They were TOP TOP predators and given little provocation they would have attacked and eaten any humans.”
What weighs against that is, when humans encounter a competitive or threatening predator species, the predator species pretty much always loses that encounter. We see it in the Americas, Europe, Australia, and continuing in the modern day in places like Africa and Asia. I guess if the groups of humans were small and nomadic, without great pressure to find a place to settle or secure new food sources, then they might just rather avoid that kind of confrontation. Basically, it might, as you say have been easier for them to turn around and go back, rather than press forward. In Europe, there were other humans pushing them on, who are of course more dangerous than the most daunting animal predators, and in Australia, they’d have to fight their way back across the the ocean and the previously settled archipelago.
So, maybe you are right, that people came and left or did not penetrate far, until later when more tribes migrating to the far east blocked their return and pushed them forward.
Funny, we used to play around Buttermilk creek as kids, making campfires and digging forts in the mud some 40 years ago and we never found any of these artifacts, but we left lots of them trying to make our own rock tools like the Flintstones. ;^)