you are aware that, with 10,000 people, it would take them several centuries at best to get back to the previous levels? realistically, they would go extinct anyway, simply because theres too many things that can and will kill humans. a demon dominated/extremely hostile world would probably put a crimp in that.
if there were only 10,000 humans, scattered throughout the entire earth, we are definitely going extinct.
You are aware about the power of a Nephalem?
In d3 it needs only one nephalem to win the fight about all lesser and big evils in one (tathamet in form of diablo).
Centuries ago, another single Nephalem was very powerfull too - Uldyssian.
Without that nephalem power, there was still other survivers and just human champions who fighted and won against diablo and his brothers or an angel called izual. Humans are not so powerless like u think.
But okay, something could happen and the human species will die. It’s possible.
No happy end for the humans in d4 → no d5?
I like the idea that the humans can lose.
…
And so the Great Conflict can going on, like it has all the time (before a Demon and an Angel falled in love).
you are aware that’s not the subject i was talking about? and, in any case, not everyone can be the top teir nephalam, or is even interested in being one. and it doesnt actually resolve the issue of having a very small number of humans or nephalam scattered throughout sancturary.
(and, mind you, they can still be killed. they are not that powerful.)
For all we know, all heroes could have been tapping into Nephalem powers.
It just needs a handfull of them to clear sanctuary and make it great again (safe).
Maybe they will start to build a wall on the borders to hell and heaven, to be sure nothing get again in sanctuary.
Could it be that after enduring all that treatment he is now stronger (intestinal fortitude)?
If he was released and had time to recover he might be looking for some payback. 
You’re correct. Approximately 90% of the human population was wiped out during the events of Diablo 3 RoS. However, even if there were little over 10,000 humans left (I honestly don’t know how many are left in all honesty); their chance of survival is still pretty high.
Not precisely. For starters, Hell, for the most part, would like to convert or corrupt humanity rather than outright destroy them as they would make perfect weapons against the High Heavens. As for hostile world, a number of different clans (some that still exist) are used to battling and prevailing against said hostility.
Highly possible, but unlike the residents of Sanctuary, real humans lack supernatural abilities.
Either way, unless either the burning hells and/or the high heavens mobilized the entirety of their forces to wipe out sanctuary; I don’t see them going extit due to an attack from either sides.
In fact, I’d even say that the truest threat to Humanity is themselves. For example, the black soulstone which was used by Malthael to dcimate Humanity, was crafted by human hands (Zoltun Kulle), as opposed to being something of angelic o demonic origin.
Honestly, I wouldn’t be surprise if a Nephalem ever attempted to craft an unbreachable barrier to forcibly prevent any angelic or demonic force from threatening sanctuary.
That would be a wonderful twist imo. I love characters who grow in strength and character by enduring excruciating mental torment.
y, that would be an easy way to explain what happend with the nephalem after d3.
Like Udyssian he sacrifices himself and his power, to build the wall. While these action, he locked the nephalem might of the humans again. And then he died.
So d4 can start without nephalem might, just normal mightless humans, which no need overpowerd might, because the doors to heaven and hell are closed again.
In an interview (when i remember right) the developers said that they want to go back to normality and away from superpower like in d3.
We will see.