While NATO doesn't have a debt account, there's no denying that the US bore the brunt of funding NATO's obligations.
It's also difficult to avoid the economic elephant in the room that most nations of NATO spent more on butter and less on guns while we were buying so much gun.
In Germany's case, there's some exception.
Right after World War Two, NOBODY wanted a rearmed Germany, even if it was partitioned into two smaller nations.
What NATO accomplished was two-fold with regards to Germany.
First it got them rearmed to be at least a credible level to assist in case War Were Declared®.
Second it did so in a framework designed to intertwine Germany's economic interests with the rest of Europe so as to make the idea of cyclically declaring war on France non-viable.
Those economic relationships still hold.
But Germany is no longer really credible with regards to its own defense. We're still there and still providing much of it.
I think it's a fine idea for them to pay the bill for that service or to stop providing it.