If the French manage a draw at Sedan, the Prussians face the reality of having to both hold there and besieging Metz, with the Austrians and - potentially - the Russians at their backs.
The Prussians and Russians had been allies of convenience at Alvensleben, but with the majority of German field forces in the west, they have a chance - along with the Austrians - of driving the Prussians back on their heels in Central Europe.
If the British decide it is time to knock the Prussians down as well - there's a reason they backed down in the Danish war (namely, no army worth the name) - but now they have the preferred British strategy to a continental power, having plenty of other Europeans willing to fight the targeted power.
Add some Danish revanchism, and hey, presto - Prussia is shoved back and the unification of Germany is delayed for another generation. Win/win for all concerned, except the Germans...
Best,