A Rhythm Heaven character isn't going to be topping the popularity charts any time soon, but they were even less requested prior to the Gematsu leak when people though the series wasn't even feasible, and yet clearly one was planned for inclusion. So even if that leak proved to be outdated, what it did do was open the gates to show people that Rhythm Heaven was, ostensibly, feasible, despite prior beliefs.
It's like how up until now, people didn't really request ATs to any great degree because they figured them out of the running. Well, if Spring-Man shows up, I bet some of the ATs will experience renewed popularity, because now the fanbase would believe promotion possible, just as the Gematsu leak showed that RH was possible.