[CarrotAndStick]

Yes, that is true. However, couldn't people have gone there and adopted local tongues?

http://thatstamil.oneindia.in/images32/optimized/vasundara1-475.jpg

[SunkenCiv]

It's more likely that technology was adopted than that language was abandoned, and even more likely that the claims are, ahem, inflated. It's also possible that it was a two way street, or even that the stuff claimed as Iranian was actually borrowed from southern India. :')