In my experience, my empire grows during the Middle Ages so that a period of anarchy to switch to another form of government is painful. It means the new government must be signficantly better than (ancient) Republic to justify the costs (turns of lost production and science).
Yes, nerfing Republic might give me an incentive to switch, but switch to what? I usually develop a few core cities, so switching to Communism always "feels" like a step backwards for those cities. Fascism means pop-rushing, which I hate. Feudalism in its present form is not better for (say) 20+ cities, with 5-8 of them size 7 or above. Switch to Democracy, and just manage the WW?
Civ4 did a decent job of reducing the anarchy penalty so that the player would make frequent civics changes. Part of that was linked to their Golden Age mechanic; a civ could have multiple GA's during a game, and changing civics was free during a GA.