^Yeah, I agree. Sakurai might want to only make characters DLC who he knows with confidence will sell very well, (such as veterans, video game icons, or very popular characters like King K. Rool, Issac, and Dixie) because making DLC characters takes a lot of resources and money. He might not want to take a risk by selling a semi-popular character to the public. Rhythm Heaven is a popular series and it has a lot of fans, but is it popular enough for it to warrant a DLC character? I think so, but a lot of Smash fans don't think so.
Hypothetically speaking, if Wii Fit Trainer had not made it into the vanilla roster, and hackers found indisputable evidence that she was going to be playable, I bet she'd have around the same amount of supporters to be DLC that most of the popular RH characters have, because even though Wii Fit is a popular and well known series, Wii Fit Trainer herself is pretty obscure to smash fans. Remember the reactions from fans when she was first revealed? I definitely think Rhythm Girl, Marshal, or the Chorus Kids would have gave fans the same wtf reaction. However, now that Wii Fit Trainer made it into the vanilla roster, her popularity has increased and by the time Smash 5 begins development, tons of people will be supporting her to be brought back. The same thing would happen to a RH character (or any obscure character for that matter) if they had made it on the vanilla roster.
So yeah, while I'd love for a RH character to be DLC and still support the idea of it, I think waiting until Smash 5 to add a RH character is probably the best thing to do from a business standpoint. But eh, who knows? Maybe Sakurai will throw us a curve ball and will give us a RH character or some other semi-popular character from the Smash Ballot.