:::@BodduLokesh, you're aware that you're "back-reading" your/our own interpretations into the intentions of the composers of the Rig Veda?: "We believe" (emphasis mine). If you want to understand what the Rig Veda meant to those people, you'll have to try to understand what it meant to them, how they used those hymns, how they were composed, etc. 
 :::@Kautilya3: yes, turning to pastoralism seems like a likely scenario - which also explains why the "Aryan"/Vedic culture had such an influence. Compare it to the Jews: the Torah reflects the lifestyle of nomadic people. Your mention of the Puranas is also interesting; the Rig Veda is not the only text which shaped the mythological properties of the Sarasvati.  
 :::Best regards,  -