Why should we assume that the Germans, if successful in the West, will then abide by any "moderate" terms they had agreed to in 1917 with Russia? (And that a separate peace in the summer of 1917 would indeed result in a German victory in the West was pretty widely believed at the time.)
Because the Germans aren't cackling super villains determined to destroy everything good, God-fearing Europeans created? Germany didn't beat the shit out of Austria again after the Franco-Prussian War, they didn't attack Russia during the Russo-Japanese War (which would have essentially been France and Russia vs Germany and likely A-H), and they didn't backstab the Italians after securing an alliance with Austria. Like it or not, Germany's behavior diplomatically was exceptional during the years between its foundation and WWI. Maybe its diplomatic strategy was stupid, but it abided by international law and acted no differently from the other powers. I see no reason for this to change after victory any more than I see a full-force Entente invasion of China, Turkey, or Germany post-war. Sure, Germany might muck around in Russia and support anti-Bolshevik groups, but I give them a .00000097973% chance of launching an unprovocted military expedition into Russia. Hell, the Entente didn't do it, and they had more resources than Germany, not to mention a member who was committed to intervening no matter what and would have relished outside support.