Zywicki is wrong. There was a great deal of corruption in state legislatures in selecting senators, especially in small and marginal states like RI, NV, WV, NJ. Railroads, banks and maufacturers bribed legislatures. Muckrakers like Ida Tarbell, Lincoln Steffens and David Graham Philips picked up on this in scandalous articles.
Populists and progressives adopted the democratic ideology of direct election. Their feeling was that the people wouldn't be bought off by special interests as legislators were. It all seems very naive now, but when you consider that state legislatures were often apportioned by county rather than by one-man-one-vote in those days, you can see why they complained. Some State Representatives or State Senators from small counties might themselves be accountable to virtually no one, and their votes would essentially be for sale in the Senatorial elections.
You could also argue that the 17th Amendment suited the ambitions of the leaders of the progressive movement. The senate was largely a backwater in the 19th century. States governments and governors had greater power then. "Senate" comes from the Latin for elder, and the idea of the senate as a group of elder statesmen persisted.
The 17th Amendment changed the role of the Senate, the type of person who became a Senator, and the public's view of the Senate. Senators became more ambitious, more hustling and more headline grabbing. They couldn't rely on their friends in the legislature but had to aggressively court the public.
Shifting to direct election also meant that every Senator could be a potential President and every Senate election a dry-run to the Presidency. No Senator went directly to the Presidency before Harding. Kennedy was the other, though dozens or scores have tried, especially since the 17th Amendment was passed.
In theory we could go back to the older system, though it won't happen. The idea of direct election has become so powerful and prominent in the public mind. It's become the source of political legitimacy. Officials who don't have that ballot box mandate take a back seat to those who do.
Were Senators once again chosen by state legislatures the Senate would lose much of its power and once again become a backwater. Congressmen would become more prominent and thumb their popular election in the eyes of the indirectly elected Senate. Governors would become more prominent in Presidential elections, though this doesn't mean that states will have any more power.
That's not an argument against change. There is something to be said for a Senate and House chosen in different ways and representing different understandings of the national interest, but it should be noted that voters probably wouldn't allow an indirectly elected Senate the kind of power it has today.
Corruption can NOT be got rid of. One can at best limit the scope of it, bias against it getting too big. The Founders understood that -- they sought only to balance it out and create smaller scale competing interest groups. It was a successful thing, at least until a constant, unrelenting motive of elitist interest drove it out.
Ida Tarbell, eh? She is a perfect exemplar!