Realtalk:
I highly, highly doubt Robin or Shulk will be cut. Robin is simply too unique and diverse to cut or outright replace. Kamui as a replacement doesn't even make sense, since the two have totally different designs and fighting styles. Chrom replacing Ike would kinda make sense, since they're both robust and noble swordfighters, but it'd still be cutting a fan favourite in favour of "repping" the new guy. I personally find that extremely distasteful, and I can't imagine Sakurai would do such a thing for any reason.
As for Shulk being cut - why? Why on earth would a popular, unique, creative and perfectly legitimate newcomer be scrapped after one game just to "rep" a newer hero, who may or may not even fight anything like him? It would be a ridiculous move, in my opinion, and nothing more than a cynical attempt at "repping" what's popular over what works. And anyway, Shulk is basically the Marth of
Xenoblade: the hero of the first (of this particular sub-series and the original one in the Nintendo canon) game in their respective series and, therefore, a representative of the series as a whole. He's not going anywhere.
Seriously tempted to start a Kamui thread now.