Your argument would make more sense, if First Class were not also the second film in which Singer tried to include Frost.
Thank god for that. As much as people complain about January Jones' acting, absolutely nothing was right about Emma Frost in X-Men Origins, acting or otherwise. At least Singer was smart enough to recognize the importance of Frost's villainous background, and the necessary role that plays before the character joins the X-Men, which is more than I can say for her appearance in the Wolverine origin movie (Jesus, Fox).
Or maybe he recognizes there's little value in Emma returning, now that Fox doesn't appear to be interested in pursuing direct sequels to First Class. January Jones didn't win over fans. There is little that is significant about the character that remains, and she doesn't offer anything that can't be found elsewhere in Days of Future Past.
Better telepath = Xavier
Better and more useful skin transformation = Colossus
True gap filler in Magneto's life = Mystique