The US has forgiven any number of debts to Mexico and Panama and who knows what other nations.
The question is not whether Geramany is effectively welching on debts. The question is whether they are legally and morally responsible for this particular debt. Now the Swiss banks are another matter altogether - They have pretty much refused to admit culpability to this day. If anyone is still on the hook for their part in Nazi theft, it should be them. On the other hand, the world of international finance is such a rat’s nest, I doubt anyone would be able to find the money in their lifetime.
Perhaps Germany is on the hook simply because it’s easier to go after them then the Swiss banking institution. Besides, who knows what the investigators may stumble upon in the process. Plenty of powerful people have a vested interest in keeping those skelletons well closeted.