Having the right team, and right budget may impact balancing issues somewhat, but they can't keep adding 15 new characters with every installment, balancing issues seem like a problem that amplifies exponentially with each new character being added affecting every other character, and there has to be a realistic limit at some point.
That is why I think we are starting to see big changes to older characters, part of what sakurai said about turning the game in a new direction, could be referring to revamping and updating old characters with each new installment.
Eventually if the fans prove to be this thirsty for new characters, they would probably have to split the game into two series. Say this game does get 50 characters, which I'm sure it won't, are we honestly going to be expecting 65 in the next installment?
I will conced that 42 seems low, but I think it is a reasonable guess, and I'd even be willing to raise my guess up to 43-44 based on some decent speculation about mewtwo and chrom, but I'm not holding my breath. We are going see at least 5-6 cuts and I think there is just no fricking way we are getting 50 characters or more.