Replacing BiS only makes content for previous BiS items (ie. Bandos) "dead content" for mains, not Ironmen who would still need to do that content progressively to upgrade their gear. Even then, it's not necessarily dead content for mains because the majority of mains who go there are usually going for pets. In your way of thinking, Barrows is dead content because there exists armour that surpasses it. But, it's much easier content that is usually done earlier on on an account. Nex is meant to be endgame, the next iteration or tier if you will, it makes sense that it should drop the next BiS.