The only way I see the war prolonged is if Germany can knock out France with the Spring Offensive, and just sit staring at England from the North of France until a white peace can be settled
I'm not so sure a white peace would be settled if France really fell, I think Germans can get good terms in that scenario. Sure, in WW2 France's fall didn't end the war, but this time France's resistance is really the heart of the conflict, and Russia has already fallen. The UK fights for balance of power on the continent and geopolitical interests so it wouldn't be happy with such a scenario, but it seems far less likely to imagine Britain and the US try to stare down central powers with absolute mastery of the continent in WW1 than it was in WW2. The logic of the Germans, at least, was that knocking out the French would give them favorable terms.
Then again, though such victory in France would quiet the home front a bit, it doesn't do anything for the great shortages created by the blockade and because of those the CP want rapid resolution. The Germans hoped earlier in the war that peace in the East would let them extract some resources from the East, but that takes time to set up and the late peace made it unworkable. Furthermore, they ended up occupying so much land that a large amount of troops had to stay East, minimizing the military gains of Brest-Litvosk. So to prolong the war, perhaps an earlier diplomatic breakthrough in the east, with lesser gains in terms of land, would work. You'd have more military power for a renewed push westwards earlier, which would logically allow for taking and holding more gains (nobody wants to surrender when the war map looks favorable), and more time to extract resources out of the conquests, which may help lessen the blockade's effects a bit.
This should add up to a slightly longer war, assuming Paris holds, which is more likely than not. It would likely mean an ever more devastated Northern France, less chance that the Bolsheviks ultimately win in Russia, and perhaps more respect for the Russian ability to defend itself in post-war Germany. Being able to walk so easily to the depths of the Caucasus after OTL's B-L didn't help the Germans in the long run. The myths that created sure played a part in making the invasion of the Soviet Union look like a workable idea.