I was just checking out gold Scythian art, and it was noted that the Scythians “just disappeared” in 200-AD.
Probably there were never very many of these nomadic peoples. They were herdsmen and horsemen and required large grasslands to support those herds. I saw a Joe Rogan podcast with S.C Gwynne about the Comanche empire which sprawled throughout much of the US Southwest. The author maintained that there were probably fewer than 12K Comanche living in relatively small bands. Then he compared that number to the seating capacity along the third base line at Yankee Stadium.
You have to wonder about the military capacity of such a small number. Even if they could put slightly more than half their numbers into their military establishment that’s still not a lot. If there are several groups with common cultural bonds orbiting around a large grassland area, it’s not hard to imaging a drought combined with warfare driving 1 of them to exit the area to prey on a farmed border area.