Modern populations apparently are not apt to reflect the ancient population of the steppes very well. Since this is large, wide open country
that during the PIE timeframe would have had nomadic "mobile" peoples
, that makes it very hard to figure out what types of DNA the PIE people had.
In this situation, I agree that ancient DNA from the right timeframes is critical. Unfortunately, the Yamnaya Horizon did not consist of a singular, monolithic people or culture ... just some shared cultural features. This means we need an adequate survey of ancient DNA from the steppes, which may be a long time coming.
That leaves us with tracing language developments and associated historical and archaelogical trails as being our only sources of evidence. This is not much different than what scientists do with atom smashers (super colliders.) They cause the collision but they can not actually see it. They are left with measuring the aftermath, but that is very helpful.
Well, I wonder if there are any correlations of IE language speakers and genetics today?