Oh come on, how can you replace Lenalee this late? She's the HEROINE. She's played a very big role so far. If you kill her, you probably kill Allen too, and then you just have a bunch of supporting characters with no leads. Sure, you can introduce another female Exorcist, but she's not going to have the same kind of impact on the plot (or on certain characters) that Lenalee did. Anyway, you'd have to make her a new Exorcist, and that would just suck. I'm sick of all these new Exorcists anyway.
...I'm also angry that everyone wants Lenalee to die just because she's going through a weak phase. No one's looking ahead. -__-;; There's no way that Hoshino is going to let her stay so pathetic for much longer; the only reason it's prolonged is because this is a long, shitty arc. Did everyone hate Allen this much when HE was weak?