The real problem lies in the numbers. Popularity kills everything eventually when it gets too popular. Think about in 1990-99 in the prime days of hip-hop, there where probably a total of 10,000 MC's globally, now every other person selling a gram of dro thinks they are thugs and rap stars, and the total MC population has risen to over 5 million at least.
When you get the popularity, you loose the individualism and rarity of the art and it gets saturated with shit. Not to mention it's 99% looks and style rather than actual talent in all genres of music it's not just hip-hop dying it's all music.
Just think of it this way if every other person in North America had a diamond studded presidential Rolex, then they wouldn't be shit because they loose the rareness and come a dime a dozen. Plus the fact that there are no topics left to rap about they have all be dissected numerous times.
Rap use to be : Original, artistic, looks didn't matter, rare, new.
Rap today : Slapped together Mr. Potato Head rappers that are generically put together by record lable A&R's, used up topics, exaggerated gangsterism to the fullest, nothing rare about it, population is 10 fold, ect..........ect.....