Originally, Senators were supposed to be a counterweight against the hordes of idiotic, uninformed voters.
The State governments would choose people who they trusted to be intelligent. Successful merchants, scientists, respected clergy, whatever.
The 17th amendment changed this so Senators were voted in directly by the people. They then brought in the 60 rule, just so the Senate wasn't subject to the tyranny of the mob.
On a pragmatic level, I shudder to think at who the majority R state governments would choose as Senators. On an idealistic level, there really does need to be a level of government removed from Trumptards or even, if you're looking at this from the other side, citizens who agreed with everything that Obama did simply because he's black.
You remove the 60 rule, the Senate becomes exactly the same as the House, and it becomes meaningless.