I think people confused "out-prioritized" with "replaced", as that's a common flawed argument towards Mewtwo and Roy.
They may not have been replaced, but they WERE out-prioritized. But then again, Jigglypuff was also out-prioritized by PT and Lucario, so that's a moot point. Jigglypuff is just different than Mewtwo in that she was able to be kept on the roster while he/it wasn't.
However, in terms of Impa, if Sheik was meant to be out-prioritized by someone, it would have happened already, since the situation for Sheik is no different than it was before, aside from OoT's remake, the summon in Scribblenauts Unlimited for Wii U, and the costume in Tekken Tag Team Tournament 2.
Midna's true form could have served a decent replacement as some people see Impa now. But it didn't happen. Why would it happen now with someone else?