Sanskrit is no older than Kurdish; English Greek etc have the same relation to it as to Kurdish — they are all alike of Indo European descent.

Like Greek and Latin, Sanskrit is an anciently written language associated with major states and ‘civilizations’ - and, crucially, is like Greek and Latin in that it is still remembered if only by some thin intellectual or priestly stratum of societies.

So by use of those three, Indo European unity could be especially well validated by pure textual scholarship (especially if the scholar was British or German and thus on another branch). It is surprising as the result is intuitively scientific — how could I arrive at it if all I do is read and write?! - and was indeed of massive importance to the framing of Darwin’s quite different scientific enterprise, which needed different evidence .