Crow, you completely ignore the "Slavic" factor in the disappearance of native Slavs
No, I don`t ignore Slavic factor. We just didn`t come to that. Factors for fall of old Slavs (ie Sarmatians) are numerous. Some internal and some external.
But, you yourself must be aware of real picture of old Europe, Eurasia and Near East, all regions which were entirely or partly part of old Slavic (ie Sarmatian) realm.
1 - Have in mind that our ancient civilization was wast and truly old. When one civilization is very old it become traditional and rarely abandoning old ways that were proven in previous eons. As such it had its own regulatory mechanisms. Some ancients lived as nomads, some as hunter-gatherers, some lived in villages and some in cities.
2 - Imagine now, when newly emerging ethoses emerged on fringes of world of ancients (in intersection of cultures), or maybe emerged even deeper in territory of ancients from within but in regions for long isolated for all kind of reasons. Imagine how ridiculous new ways looked to our deeply traditional ancients. Some were faster to change and some slower but in general they were reluctant to change. For millennia they lived, ruled their realm, spread and dominated and now some new people coming and endangering them. Imagine psychological impact of that situation.
3 - Then think of all new factors that befall our ancients... one of them era of slavery..... Greek and Roman slave oriented society. What you think what was reaction of our ancients? Two kind of responds. At firts they were deeply shocked that one human can take other human as property and later with time, they succumbed to the influence and some, entire tribes, oriented to live and prosper by hunting their own brethren and selling them to Greeks, Jews, Arabs and Romans. So, at first strangers used to hunt Slavs (ie Sarmats) and then some Slavs started to hunt their own and profit on it.
4 - New religions spread by Greeks and Romans were also plague for our ancients. Especially Romans harmed them with invention of Christianity and then purposely segmented Europe on Western and Eastern Roman of influence. It was first time in history of ancients that they faced religious turmoils. Until Romans they were tolerant and have numerous deities from one side of their realm to the other but, with Roman influence and Christianity they started to killing each others.
5 - Have in mind that Romans invested great effort to weaken cohesion of our ancients, even in time BC and later in Christianity. At first, hunt of ancients was openly justified with slavery and necessity to acquire populace, while later, same reasons were justified with Christianity. So entire crusades were initiated against against non-Christian ancients.
6 - Complete situation led to total confusion and collapse of values in societies of ancients. Numerous invasions moved those who survived on internal migration. Those were cataclysmic migrations. What you think how sedentary tribe was happy to move on territory that was traditional zone of living for some nomadic tribe? Or when nomads were forced to settle fast, losing territories to feed their flock of animal? Or when some tribe in fear from Christian armies accepted Christianity and started to pressure other still non-Christian tribes, that they too accept Christianity? Imagine wish of retribution in those who were stubborn to give up from old Gods? It was era when treason was reasonable in order to survive and when heroes of some clan/tribe were hated by their own kin.
And on all what was happening to world of ancients, foreigners profited.