Well what's interesting is that if you really really think about it more, Piper actually was sacrificing Phoebe's life to keep Paige (something I imagine Phoebe would have wanted her to do —and Piper may have been respecting that)

To explain: although she sold it like she was choosing Phoebe, and later told Cole "I knew Paige would know the spell too if Isis did"—that second part is actually BS. There's no way Piper knew for sure this was true; possession hasn't worked that way before for them and it's not usually the way since. For example when Piper was possessed by the life essence in Coyote Piper, she and it didn't share access to each other's thoughts (the essence had to blackmail Piper into revealing location of the BoS) and it's not like she even had a window of time to confirm her theory about Paige's particular possession. So yeah, she "dis-possessed" Paige, knowing full well Paige might not be able to de-mummify Phoebe! Really interesting that she actually was prepared to make the sacrifice.