Well heck, why didn't Mitchell and the rest of the Dems propose ending the filibuster back then? Because as Mitchell correctly states:
During my six years as majority leader of the Senate, Republicans, then in the minority, often used filibusters to achieve their goals. I didn't like the results, but I accepted them because Republicans were acting within the rules; and we were able to work together on many other issues. There were 55 Democratic senators then. We had the power to take the drastic action now being proposed, but we refrained from exercising that power because it was as wrong then as it is now.