The level of ignorance and pretentiousness in this sentence is mind boggling. The human voice is the ultimate instrument, not the piano. Hell, we have not even had the piano for that long. Words can not be expressed on a piano. That is what makes singing and rapping unique, you are able to tell a story combined with music. Strange Fruit would not have the same meaning or significance without the words attached to it, neither would many songs.
Also, Miles Davis himself loved and embraced rap. He literally made a jazz/rap album.
Jazz is a dying genre in the united states, because American audiences are tired of hearing it, and there have not been any groundbreaking new jazz artists in the last 30 years.
Rap is one of the most interesting and innovative genres of music now, and literally incorporates and uses so much jazz. To Pimp a Butterfly is a great example of a rap album that is heavily influenced by jazz. It was also a wildly popular album and one of the most critically acclaimed rap albums ever.