I don't think democracy is about having the "best" government, that'd be technocracy, I guess. Democracy is about sharing the power, and having the most legitimate government based on the principle that everyone's voice is equal.

Also, it's not a zero sum game at all. First, even if the long stretches of good and bad governments equalized over the long term, it's much worse to live all your life under a bad government than have one half the time. Also,thinking as an engineer, think about the stability implications of a wildly swinging signal vs a smoothly swinging one, even if both are centered.

In any case, coming from a country which had a long dictatorship until not so long ago, it's fairly clear it's not a zero sum game.