I know we needed the Empty to have beef with Cas so one of the trio had another awful deal shoved on them and so he could die dramatically for Dean the following season but can you imagine if it went the other way around?
Like, the Empty just wants to sleep and Cas is annoying and noisy, why would it want him there? What if instead he just got fucking banned from angel afterlife?
Imagine the Empty coming for Jack in heaven and Cas tries to fight it and the Empty is just like "fuck! the annoying guy is here, pretend you didn't see me, gotta flee before he starts talking to me" and they get Jack back genuinely for free
Imagine Billie getting into that room where this time Dean and Cas have no plan more than going down fighting and she goes for Cas and after mortally wounding him she snaps him away to the Empty like she did that one time with Jack, so there's no body and she turns to Dean who is trying to grapple with his shock and grief while also wanting to kill her for this but then they hear something colliding with the shelves and Billie turns around and Cas is there again, alive and looking like he just got thrown very hard so she's like "this is impossible, how did you do it?" and Cas just blinks, bewildered at what just happened to him "it seems I am too annoying and have been banned from dying"
So Cas just keeps shielding Dean from every attack and the Empty gets increasingly annoyed until it's throwing Cas straight against Billie so she will just stop sending him and... she dies, eventually, because Cas is literally too annoying to die