Why do you think Senators are elected every six years, instead of every two years like the House? The Senate was a nod to the British crown's parliamentary style of government. A check on the House.... especially as it pertains to emotional and/or hot button issues.
The people elect members of the House from the districts in which they live. Senators were chosen by the states to represent the entire state that they were from. It also would serve (similar to the Electoral College) the purpose of insuring that a handful of largely populated states did not effectively control the rest of the smaller less populated states. That's why there are only two Senators for every state in the union.
Direct popular vote for Senators effectively neuters the original intent of check and balance intended by the Founders. Additionally, why do you think that all spending has to originate in the House of Representatives?
Presidents often get the blame for spending.... it is the nature and risk that comes with being the Chief Executive, but the President can only propose. It is Congress (specifically the HoR) that disposes.
I think the Founders knew that once people figured out that they could vote themselves bread and circuses, the game would be over.
They put enough checks in to make that mindset hard to reach, but ultimately, it comes down to a moral and civic responsibility on the part of the voters.
We can all sit here and blame politicians until the chickens come home to roost.... but the voting public is the problem. The politicians reflect that image.
Democratizing the Constitution:
The Failure of the Seventeenth Amendment