That’s exactly what they realized during season 2 of Stranger Things, which is why Eleven and Hopper went to the lab to seal the gate: they needed to cut off the connection between the Mind Flayer and its demagorg subjects. Beric Dondarrion (Richard Dormer) came to a similar conclusion in season 7 of Game Of Thrones after realizing that killing a White Walker led to the death of all of his wights. We end the season with the hope that killing the Night King, who created the White Walkers, would end the army once and for all.
