I think it's a little trickier than standing in front of a microphone and reciting rhymes to a beat. For every so call "rap star" there are thousands who aren't able to break through selling millions of CDs.
Based upon your analogy any attractive girl could make a sex tape and become the next Kim Kardashian! It doesn't work like that.
There is always an "X- Factor" that causes audiences to (choose) one entertainer over another.
Thirty years is nothing when you consider how long country, blues, jazz, R&B, and rock music have been around.
I suspect many folks thought rap or hip/hop music would go the way of Disco. Have a few "hot years" and then fade by the wayside.
I think the difference has been how hip/hop music artists have penetrated other areas (movies, fashion, fragrances, shoes...etc)
James Brown, Aretha Franklin, Elvis Presley or Janis Joplin never conceived of creating a "brand" along with synergistic businesses. In fact most of those artists back then didn't own the publishing rights to their songs and primarily got ripped off.
This sense of entrepreneurship in the hip/hop era has been one of the biggest differences. Dr. Dre sold his Beats Audio headphones to Apple for $3 Billion! Another factor has been the tie in with sports teams and athletes. A lot of kids want to emulate their sports heroes and if these people wear the products of hip/hop artists or listen to their music they are likely to follow suit.
Everyone wants to be seen as or thought of as being "cool".
Last but not we live in the social media age.
One doesn't have to rely on MTV to get the word out. Some recording artists such as Justin Bieber and Ariana Grande were supposedly discovered on YouTube.
Having millions of followers or going "viral" can change your life!
However as I mentioned not everyone makes it who tries.
"Many are called but few are chosen." Matthew 22:14