Smaller reproduction bases....deaths outnumbered births. Pretty simple...
Note the time span they share about how long it actually took for it to happen? What you say makes very good sense considering the actual time span. It didn’t just happen overnight as assumed.
High birth rates for every race but white and Asian will cause those to soon become extinct. Then disease and nukes will wipe out the darker pigmented races.
Exactly what happened to the Spartans.
Overall numbers shrank till the could not send out a proper army
Then again, maybe todays arabs are yesterdays neandrthals