That's two characters at best (assuming Ike returns and Chrom/Roy/etc. get in), if we were to scale down to a ~30 roster that's not going to cut it. We have along with the main characters a cast with others like Peach, Bowser, Zelda, Ganondorf and the like, there's already going to be a good chunk that has every reason to stay in the cast, moreso then those two. Plus, we have to consider new additions from other series and third parties.
Trimming the fat isn't a pleasant process, but one could be lost in such a situation.