if one population became another population and then another population there still had to have been a time when an outside observer would have said "there is a human". all the others were still apes. so to say there were never less than 10,000 HUMANS still does not make sense because the changing populations were not humans. when that first human appeared there was only one not 10,000
I'll just assume your version of these words to better explain it.
As populations very gradually adapt and change into other populations over time, there isn't one specific birth that delineates the dividing line between the two populations. One group of advanced primates doesn't sudden;y give birth to a new group who will go off and become a new species within 3 weeks. That's not how biology works. Just like two North Europeans aren't going to give birth to a child with Sub-Saharan African features, so one population won't spontaneously produce an entirely new population. It takes time and environmental pressure to make that change. It's no different than you having family spread all over the world that you have no idea that you're related to. You probably have cousins in the very town you live in that you've never met. You have grandparent's whose names you've never heard of. You no nothing of their life story. Nothing at all. Over long enough periods of time, the offspring of the people who are your cousins now will be so far removed from your bloodline that you will no longer consider them part of your family, right? See how this works? That exact same concept, spread over incredibly long periods of time results in not just very different physical features between you and the people who you were once related to, but it can produce entirely new species.
Understanding this, and knowing that all complex living organisms had to have parents of some kind, you can see how one population slowly changes into another population. 10,000 humans didn't just pop into existence out of nowhere. They came from very similar preceding populations, which came from very similar preceding populations and so on. No one is saying that one day a group of Capuchins gave birth to a group of Orangutans who one day gave birth to humans and there were 10,000 of them... That's not at all what's being said.
By this definition, 10,000 breeding members of a population who could or would go on to produce modern humans have, so far as anyone knows, always existed.