Hell, producers have been just as, if not more important than rappers for like the past 20-30 years. I mean, between 2000-2005 I'd argue you couldn't go more than 15 minutes listening to an urban station without hearing The Neptunes or Timbaland, and everyone knew who they were. And when it comes to R&B, I'd argue that the the majority of the performers since the creation of the genre were almost entirely just singers, which is what makes the virtuosos like Curtis Mayfield, Isaac Hayes and Stevie Wonder stand out. Just using your Motown example, there's
The Corporation and
Holland-Dozier-Holland as the creative forces behind the music.