The truth is the two are similiar but they are not even close that Rachel could ever replace jean. The two are very separate characters. One is a woman who always knew herself and the other is just trying to exist and find herself.

Rachel isn't Phoenix or really that powerful, It becomes clear when you look past her powerups to what she does. She's never been uber powered or a Jean wannabe. She lives in a constant shadow and she's just trying to find her way. I love her character. jean was self assured from the get go but Rach. is a work in progress.

Its only becoming a problem now that Jean is gone. Seriously if Jean was around would this even be a topic. Oh and with everything Jean being kiled of I think is good to have a reminder. Rach is the only one who even tries to remind people that Jean Grey existed and that she should come back. Even Scott has stopped saying her name.
Fire Made Flesh