Claiming that Thalyssra used the withered to, “further her own ends,” is incredibly disingenuous, especially considering that Thalyssra’s goal was only ever to save the people of Suramar.
Thalyssra’s harnessing of the withered ensured that they served a higher purpose other than being some mindless fiend terrorizing the Broken Isles or fodder for the Ettins and Fal’dorei to kill or feed off of. Thalyssra gave purpose and meaning to a people too far gone to cure, and allowed them with the last of their life, to fight back against the forces that inflicted that fate upon them.
The Nightborne are indebted to the withered - it was through Theryn, a withered, that the Nightborne discovered the Arcan’dor, the means by which they were able to end their crippling dependency on arcane energy and would ensure that they would never know the fate of withering again. Even after Suramar’s liberation, Theryn is allowed to roam the gardens of the Nighthold until the last of his days, in peace. 
None of this was done callously on Thalyssra’s part or to fulfill self-serving motivations.