I'm definitely one of Krakowski's biggest fans, but she her dancing skills are nowhere close to Cassie's. Also, she would definitely throw off the balance of the show. Krakowski is considered a Broadway "star," I don't think Cassie should be played by someone who will immediately be recognized as a star (though this worked for McKechnie her second time in the role but I think that is different since she had been connected with the show for such a long time). What it comes down to is that she could never get through the Michael Bennett choreography that even trained dancers who have constant training have issues getting through.
Amy Spanger would not only have issues getting through the extensive marathon dance sequence but I can't see her acting being genuine enough to make the scene work. I can't believe people who criticize Charlotte would suggest Amy Spanger of all people.
Natascia Diaz, Rachelle Rak, and Shannon Lewis would all be top-notch replacements for d'Amboise when she decides to leave the show.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"