You should read the scholarly papers by Todd Zywicki in this thread, Repealing the Seventeenth Amendment, especially this one. It lays out the common reasons why the 17th passed, and then tries to disprove them by pointing out incongruities in the arguments, and then finally explains a market within the Senate for trading political votes based on longevity (the ability to back up the promise of future votes due to the guarantee of being there).
You might even think of this as the foundation for the creation of the Ruling Class as depicted in this lengthy essay.
The bottom line is that the original system wasn't corrupt enough, which is why the 17th was passed -- to give Senators (as power brokers) the ability to promise to deliver legislation because of the guarantee of remaining in the Senate long enough to make good on trades for votes.
-PJ
Repealing the Seventeenth Amendment
-PJ