But there was an undercurrent that even the author of this piece misses, and it explains the interconnectedness of the 16th & 17th Amendments and even the Pledge of Allegiance.
Two lessons came from the Civil War.
A de-emphasis of the states was necessary. Sen. Edward Baker of Oregon had suggested abolishing the states just after Fort Sumter, but his death in 1862 in a botched retreat at Ball's Bluff stilled the voice of what would have been the most radical of all Radical Republicans in the Reconstruction era. While Baker's suggestion went too far, even for Lincoln, the Progressives were interested in finishing the work of Hamilton, Clay, Webster and Lincoln by placing a constitutional emphasis on national sovereignty at the expense of state sovereignty.
The 16th Amendment ended the whole "tariffs and imposts" issue by letting the federal government tax people, not just things. This direct collection of revenue, bypassing the states, created the basis for the ultimate goal of the Progressives, the New Deal.
The 17th Amendment de-emphasized the states in favor of direct democracy. Direct election of senators should be placed historically next to other Progressive goals such as the secret ballot, open primary elections, and initiative, refendum and recall. All these were intended to emasculate the states and lead to direct democracy, rather than a federated republic.
Even the Pledge of Allegiance, a product of the Fabian socialist branch of the Progressive Movement, was created to replace the Constitution with the Flag as our unifying icon. It was all a part of a general thrust in American history, a reaction to the bloodshed of the Civil War.