@Young,
Everything is relative. I wouldn't call the Yamnaya in even modern Greeks very significant...They Mycenaens had about 13%...they don't look to have gotten all that much more from the Slavs or other later migrations, although they got some (and these are mostly or all mainland Greeks)
Just a reminder that this graph is
not based on a simple admixture run.
What is inescapable, imo, is that the processes that affected the Balkans, Greece, Iberia, (and perhaps Italy, although we have to wait for adna) are very different from what happened in central and even more in northern Europe (and perhaps in India). The same seems to be true for the Near East (i.e. Hittite and other Indo-European languages.)
In these latter areas the Indo-European speakers entered densely populated areas with advanced cultures. In northern and central Europe the territories were either empty or relatively depopulated because of population crashes caused by climate change, environmental degradation of the soil, and perhaps disease brought by the newcomers.
As a result, the total effect genetically in the Balkans and Greece was much smaller (75% replacement in the north originally, versus either 13 or 21% in the Mycenaeans depending on the group being examined.) Culturally there was a big difference as well. The newcomers in Greece, while changing the language, adopted a lot of the culture.