Real R&B has always struggled on the mainstream charts compared to plain ol' pop. It was usually the most commercial, mainstream, water downed, glossy R&B music that could be more easily marketed to pop audiences that tended to break through. Back in the day, more soulful R&B acts like Gladys Knight, Phyllis Hyman, Gerald Levert, Aaron Hall, After 7 and Johnny Gill usually didn't get strong airplay at pop radio or chart high on the Hot 100. They may have score a small handful of hits that charted decently at pop, but the bulk of their material was urban supported only. There were some exceptions like early Toni Braxton and Alicia Keys, but they weren't the rule. Even in the 90s when R&B was at its commercial peak, there were many many R&B artists and songs that were washout's at pop radio because they weren't mainstream enough.

If you really wanna know what killed R&B, it was hip hop. Around 2002, hip hop became more important at black radio and R&B got pushed to the back burner in favor of it. That's the reason R&B hasn't evolved much in the last 10 yrs. Because the urban music world cares more about hip hop than they do R&B and so R&B gets left languishing without enough people focused on pushing it forward. Then pop came along in the late 00's and made that lack of support even worse. In order for R&B to come back, it needs to become more important with it's core market (black people and black radio.) From there it'll snowball up and regain its foothold. But I don't know if I easily see that happening, because now there's a whole generation of young people who only know things as they've been in recent years and have no desire to see things return to the way things were before they were born or started really paying attention to music.

Pop trends are very fickle. What's hot today won't be what's hot 5 years from now. Dance and pop will die out eventually. Mark my words. People will turn on this peppy upbeat "happy" sound in favor of something more "serious" and a backlash against this euro, techno, house, bubblegum, dance sound will be in force. The bigger it is, the harder it falls. So it'll be a nasty death when it finally happens.