I always just seen the warband are just alternative sculpt that you can use as proxy and never meant to refresh or act as replacement for older models. it not like you can viable buy enough Hrothgorn mantrappers to fill out enough Sabre tusk for your army, same can be said with those Skaven gutterunners too. Even if certain ones like Zarbag is an updated sculpt for a outdated madcap shaman model, he is definitely not the official replacement of that said model at least in GW minds