I think OP's point was that more directly transporting water to where it's used by humans is more efficient, and will also remove/reduce that particular consume

I think OP's point was that more directly transporting water to where it's used by humans is more efficient, and will also remove/reduce that particular consumers impact on existing rivers (therefore reducing impact on ecosystems). To more directly address the OP's point - dumping water into existing resevoirs and rivers allow us to reuse existing infrastructure getting water from source to consumer (at the cost of other inefficiencies). I'm certain the real answer is case by case and if such a system were to be implemented, we would see the whole spectrum of implementation deployed.