A thought about national debt

For as long as I follow politics here in Germany, I hear that we have to lower the national debt in order not to leave too much of a burden to the next generation. As I understand it, this is also an issue brought up repeatedly in American politics as well, and surely in a lot of other countries.

However, when the next generation pays back national debt, say in 50 years - aren't they paying back to people who are also of the next generation? So isn't national debt a zero-sum game generational-wise by definition?

Thoughts?