Poland's international position by annexing East Prussia would greatly strengthen her by allowing her to utterly dominate the southeastern Baltic coast and all the trade that comes with it. It basically eliminates the massive Brandenburg-Prussian knife in their ribs and makes it easier to secure her borders. This much stronger Poland would find it in her best interest to keep the German states disjointed and disunited, playing sides off one another. Even without having to worry about Prussia anymore, there is still a growing Habsburg presence to the west, Russia to the east, the Ottomans to the south, and Sweden to the North and across the Baltic that the Poles have to worry about. Having East Prussia in the PLC strengthens geographical cohesion for Poland, but one thing they cannot allow for is for any significant German unification to further happen.
If any desire for unification happens, it likely happens further down the line and in a way that would be difficult to predict. I generally see a handful of independent German states arising over time, with the smaller ones all getting snapped up or annexed. Austria, Bavaria, Saxony, Brandenburg, and Hanover I could see becoming modern day nation-states, speaking closely related German languages, but languages that evolve in their own way.
And of course, France always does better when the Germans are disunited.