Tacitus owes Kashub anything
Apparently you fail to grasp the idea of legal continuity. Tacitus may not owe Kashub anything, but at the same time Germany can owe something to Poland. Modern Germany is the legal heir of the Third Reich and Germans build their image of a "moral superpower" on the fact that they admitted the crimes of Third Reich and accepted their consequences.
As far as the issue in question is concerned, Poland can take two approaches: moral approach or pragmatic approach. Let's take a look at both:
a) moral approach
German reaction to Polish demands is somewhat disgusting (at least for people with weak stomachs) - on the one hand we have undoubted fault: unjustified aggression, millions of slaughtered civilians, country in ruins, works of art stolen etc. etc. and on the other hand legal tricks and small print from the times when Poland was a Soviet puppet-state (and even then, questionable legal claims). Basically, what Germany is saying now is: "Yes, we take the responsibility for III Reich's crimes", so Poland says "Well, then pay up", to which Germany replies: "No, no - we take the responsibility, BUT we won't pay - we don't have to pay because of X*". Absolutely indefensible approach and morally vomit-inducing. In this scenario, Poland cools down the relations with Germany, maybe even to the point of Polexit, seeing that Germans still think it suitable to block Poland's European funds and try to meddle in affairs not covered by European treaties (e.g. much needed judicial reform) and feel it also suitable to lecture Poland on democracy and European values (!). An unlikely scenario? Perhaps.
b) pragmatic approach
Germans are still Germans. People don't change that much in 75 years and certainly not nations or countries. They are still the 1939 Germans** but, luckily, they are much more pragmatic now, so maybe we should adapt their approach too. Sure it's unfair, sure it's disgusting, but for an average Jan Kowalski Poland in the EU, strong economic ties with Western Europe and wide cooperation with Germany is much more important than moral high ground and satisfied righteous indignation. So, if Germany gives up the meddling in Poland's internal affairs, we should adapt the pragmatic approach and keep the widely understood cooperation with Germany as an important part of the EU, and put the issue of reparations ad calendas Graecas
. An unlikely scenario? Perhaps.
*X = legal tricks and small print;I would be inclined to adapt approach B.
Having said that, German reaction surprises me. I would expect at least some sort of negotiations; certainly not the quick and cold "nein" that we heard. Surprising (or maybe not - see **). Oh, well.