"just give it another 50 or 80 thousand years and there will be multiple species of human."
That isn't as clear as you might think. At the moment, we've returned to the entire human species living in one gene pool, one that spans the entire globe. We'd have to lose the ability to cross the world in timeframes on the scale of a human lifetime, and even if you postulate a total civilization collapse and a total loss of all knowledge, we know that civilization could return to an Age of Exploration-level of capability in a mere few dozen generations even under the absolute worst case scenario that still has humans left to talk about at all. Speciation under such circumstances is certainly not automatic.
However, talking about human speciation without accounting for the extraordinarily high chance that it will end up being driven by human intelligence is probably a waste anyway. Again, on the tens-of-thousands-of-years time frame, even if our civilization completely and utterly collapses, another one could arise on that time frame quite comfortably. (Civilization #2 faces some resource issues, but with enough time they can still be overcome.)
You can look at history as well. Australian aboriginal homo sapiens were isolated for 40,000-50,000 years or so without any speciation.
You'd need an isolation event and a VERY long period of time; we have much more control over our environment than the aboriginals, which would probably serve to minimize differences in selection pressures between isolated populations to a far greater degree. 50,000-80,000 years doesn't come close to long enough, IMO.
If brown dwarfs and giant wandering planets are as prevalent as we think they are, then there's almost always a port of call for humanity only 2 LY away for large swathes of the galaxy. If we can figure out how to create "seeds" of our civilization that are reasonably compact, we can send them across that distance in only a couple of decades using laser sails and other propulsion schemes not limited by the rocket equation.
That isn't as clear as you might think. At the moment, we've returned to the entire human species living in one gene pool, one that spans the entire globe. We'd have to lose the ability to cross the world in timeframes on the scale of a human lifetime, and even if you postulate a total civilization collapse and a total loss of all knowledge, we know that civilization could return to an Age of Exploration-level of capability in a mere few dozen generations even under the absolute worst case scenario that still has humans left to talk about at all. Speciation under such circumstances is certainly not automatic.
However, talking about human speciation without accounting for the extraordinarily high chance that it will end up being driven by human intelligence is probably a waste anyway. Again, on the tens-of-thousands-of-years time frame, even if our civilization completely and utterly collapses, another one could arise on that time frame quite comfortably. (Civilization #2 faces some resource issues, but with enough time they can still be overcome.)