[William McKinley]

The constraints put into the Constitution against changes to the Constitution were not sufficient to prevent changes to the Constitution such as the 16th and 17th amendment.

But that has absolutely nothing to do with the arguments between the Federalists and the Anti-Federalists (except for the fact that the Federalists thought that the amendment process might allow 2/3 of the states acting together to subvert the Constitution. In other words, the Federalists wanted to make it tougher, not easier, to alter the Constitution).

[William McKinley]

And who was it that proposed and pushed for Congress to be able to propose amendments instead of only state legislatures? Madison, of the Democratic-Republican party, not the Federalists.

And who was it that proposed for there to be provisions for new Constitutional conventions to be called? Elbridge Gerry, of the Democratic-Republican party, not the Federalists.

If the provisions for modifying the Constitution were too lenient, blame the anti-Federalists, not the Federalists.

Personally, I do not think that the amendment process is too lenient. I think that when the 16th and 17th amendment were passed, the problem was that effective counter-argument against them were not made. The system didn't fail the country, the country failed the system.