http://en.wikipedia.org/wiki/Substratum_in_Vedic_Sanskrit
"A concern raised in the identification of the substrate is that there
is large time gap between the comparative materials, which can be seen
as a serious methodological drawback.[4]"
"Substratum vs. adstratum
Hock 1975/1984/1996 and Tikkanen 1987 (as cited in Bryant 2001:8082)
are quite open to considering that various syntactical developments in
Indo-Aryan could have been the result of adstratum rather than the
result of substrate influences.
About retroflexion Tikkanen (2005) states that "in view of the
strictly areal implications of retroflexion and the occurrence of
retroflexes in many early loanwords, it is hardly likely that
Indo-Aryan retroflexion arose in a region that did not have a
substratum with retroflexes."
M. Kelkar