I already addressed this: 1. Artists can range from pop to hip hop on a spectrum and there is no clear line where one begins and the other ends, so even Soulja Boy, if he tried, could make a hip hop track and he could even still go back to pop after that just like a Jazz artist can make a funk song and go back to Jazz.
2. Pop isn't necessarily bad. For example, I consider "Rapper's Delight" to be more of a pop song than a hip hop song and it is more pop than "My Philosophy" by KRS-One, but even though I like KRS's lyrics better, I preferred the track "Rapper's Delight" overall. That doesn't change that it is more of a pop song.
3. I already stated, some underground artists are just as much if not more pop oriented than these mainstream artists. Mainstream does not equate to pop/commercial and pop/commercial does not equate to mainstream.