Imagine a Prussia that doesn't hold Westphalia or the Rhineland. In fact, it doesn't hold anything west of the Elbe.
It does, however, hold Silesia, Bohemia-Moravia, Saxony and Poland ("Poland" here means OTL Prussian partitions of Poland + 1795 Austrian partition).
Can this alternate Prussia hold it's own economically? It doesn't have to lord over the other German states (in fact, it should stay away from any German unification projects), it just needs to be self-sufficient enough that it won't be pulled into the economical orbit of a "Germany" (like what happened to OTL southern German states).
Industry aside, what about agriculture? The Cleves-Mark area was Prussia's most fertile territory. Can Poland provide similar crops, or is it just more Prussian sandy wasteland?