Seems to me rap has just entered into being pop music.

You must be forgetting J Lo's early collaborations, and P. Diddy's solo debut and all that. Common released a song about how hip-hop is being ruined by commercial sell-outs way back in 1994, which probably a lot of us consider part of the Golden Age. The way I remember it, I stopped seeing adventurous and groundbreaking hip-hop get regular play in mainstream rotation right around the time everyone started competing for "Most expensive video ever made," things shifted when it became firmly style over substance. Credibility used to be everything, skill used to be everything... then getting a helicopter and a yacht in your video was everything.

There've always been guys like Slick Rick rocking massive chains and all that, but there was cutthroat competitiveness when it came to writing and producing back then, that's why there's a Golden Age to speak of. But a lot of remembering it as the Golden Age is the amount explosive creativity that actually got mainstream airplay and succeeded commercially. Subsequently, you got label executives attempting to capitalize on that success without understanding why it succeeded, and then you get years of derivative crap filling the void.

Happened to pretty much every genre in the back-end of the 90s/early 2000s. Then the whole distribution end of things changed and music video television stations died, and the possibility of a mainstream packed with creativity and diversity of sound kinda went with it.