libertarians: should us senators be elected by the people?

[Verily]

But the worst constitutional amendment on the books is the 16th which created the income tax.

The Sixteenth Amendment had no effect whatsoever, except to clarify what was already implicitly provided in the original Constitution: income taxes are permissible.

Prior to the 16th amendment there was no income tax except for a brief period during the civil war. After the 16th amendment income taxes became the primary source of revenue. So it very definitely had a profound effect.

Constitutional amendments are difficult to pass. Why would government do something so difficult if it was not necessary?

Legally, it wasn't necessary. However, it was useful to prevent costly and futile challenges in the Courts.