In human contexts even 5000 years is forever. I know for a fact that the indigenous people of the Americas were here before that. There is disagreement as to when, but not if. There are archaeologists who have truly strange religiously driven objections to when humans arrived in the Americas but mostly people want evidence. That 15000 BP number is pretty solid and you can say yes it happened, things before that start to look like special pleading. When you accept the special pleading for one case you'll have to accept it for others as well.
Try this example on, there is huge funding to prove the accuracy of the accounts of the Bible. Apparently people believe this will prove the existence of God. If you accept their special pleading you'll have to accept that the archaeological evidence for the Trojan war is stronger and they have written accounts as well. Using the special pleading, doesn't that prove the Greek Pantheon exists?
The indigenous people of the Americas where here so long ago that it is forever. If you doubt that consider that English as we know it has only existed since about 1580, and we'd have a hard time understanding them. Additionally there is no evidence that the people we call English were there 10000 years ago. People yes, English? Define that for me, they didn't speak what we'd call English so you'd have to say no.
They have to be conservative because what we are learning about American prehistory will ruffle someones feathers, be it Native Americans or biblical literalists who can't really explain how they got here at all, but insist the C14 dates are bogus and must have been contaminated.
This is a disagreement that has gone on for decades, and as with all science extraordinary claims require extraordinary evidence.
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