Maybe the purge is something that the SOV player needs to do, otherwise there's an increasing chance of civil war every year? That way if the SOV player puts it off, the game balance remains in play because if GER makes good progress and lowers National Unity, the SOV player may have a rebellion on his/her hands despite having a better officer pool.
Despite the possible historical realism of it I think it's better with rewards as in more Political Power points making some historical national goals easier to reach than if you forego the purge.
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