“The appointed Senators were the protectors of the 10th Amendment.”
Were they really? I think the income tax amendment was the big change. Millions more for the federal government. A useful experiment would have been to space out the two changes and see how protective of state powers senators elected the old way would have been once they got all that money.
And other people would say it was the Federal Reserve that was the biggest change.
Unlimited money creation, just as the US dollar became the international currency.
Fact is, all three were important, and each became more significant because of the other two.