The other thing is that, by and large, removed Firstborn Marines usually get a Primaris equivalent eventually, if they don't already have one - so it's less "they're squatted" and more "they have better rules and better models that play basically the same but have a different name".
So in the case of the Wanderers, I'd be a little less miffed if we had lots of Kurnothi out or clearly on the way, and a *lot* less miffed if we had new aelf units actually in Cities of Sigmar that filled a gameplay role where the old Wanderer models could be easily proxied in.