One reason why the amendment was passed was that state legislatures were regarded as corrupt, and the election of Senators as particularly corrupt. Of course corruption still exists, but remember that these were the old pre-reapportionment legislatures in which one or more sparcely populated counties might have as many votes as Hartford, Cleveland, or Atlanta. Had reapportionment come first, it's just possible that indirect election could have survived the storm of the Progressive onslaught, but one or the other was inevitable, and the courts hadn't taken yet the power to enforce one man one vote.
Also, a little money went a long way in Nevada, North Dakota, or West Virginia. Farmers in some states complained that the railroads had more influence in selecting the Senators than they did. I suppose a little money still can, comparatively speaking, buy a Senate seat in a small state, but the money goes into television advertising and not into direct bribes. Actual bribery offends in a way that today's campaign contributions don't.
Zywicki's theory has a lot of problems, but I think he does carry one point. Senators themselves, those who survived reelection, grew in stature after the amendment. Every Senator became a potential President, and every senatorial election became a dry run at the presidency. This wasn't the case before direct election. And though it seems like almost every Senator has presidential ambitions, only two Senators have gone directly to the Presidency, both since the 17th Amendment -- Harding and Kennedy -- and their careers show the qualities that voters have selected Senators for: good looks and smooth, ingratiating manners. Direct election was in the interest of the Senators who would survive it and of those politicians who thought or dreamed that they could win Senate seats for themselves.
For the Progressives, popular election was the source of all legitimacy. They would have by-passed and watered down the Senate's powers if they couldn't win direct election. And that's probably the chief criticism of repealing the 17th. If Senators were again indirectly elected, they'd have to take a backseat to the directly elected House and the nationally elected President. Given the way people have thought since the Progressives, an indirectly elected Senate -- like its counterparts in Britain and elsewhere -- would find itself marginalized and stripped of power. On the other hand, we wouldn't have so many Senators clamoring for television time and scheming to become President.
One idea that came up on the previous thread was increasing the size of the House to 1000 or 3000 members. Given such a large body, elections would cost less per seat. Big contributions and television money would matter less. Representatives would be closer to the people and have fewer perks. There would be fewer career legislatures and more openings for outsiders. At least that's the theory. It seems to be born out by the experience of some of the older states that downsized their lower houses and found politicians becoming more professional, more ambitious, and more favorable to higher taxes.