The Arbiter sorts all dead souls from every planet in the cosmos, as well as every planet in every alternate reality. The sheer amount of anima flowing into the Shadowlands is staggering.
How many people actually died at Teldrassil? A few thousand? Maybe a hundred thousand?
The drought is on the scale of multiple universes. A few thousand souls - even the best souls from Elune's chosen people - are not going to stem the drought in any meaningful way.
This entire cut scene seems to be created to make help minimize Sylvanas' atrocity.