I think Stuff made a decent point.
As to the Germans choosing to be more like Belgium and not pony up: not too surprised.
It is a lot easier to sell a program of collective defense to your political public than it is "to go elsewhere with our blood and treasure" ... which may be part of Merkel's problem.
The German political landscape is also morphing.
During the Cold War, we used to see this mutual eye gouging between the US and a wide number of NATO allies on who was or wasn't showing up to fill the buckets. A few years back, I was reading a biography of Ike and was interested to find out that this was a problem in the mid to late 50's as well. I'll make an estimate that it's been a source of political friction since the Alliance was formed. (And a few nations show up regularly and drop the money in the bucket)
There's been an ongoing political debate since about the time the Wall went down: whence NATO? Do we still need it? In the mid 90's, somewhat to my surprise with the "peace dividend" and Asia becoming a major area of strategic interest (Pacific Rim model). the vote remained "Yes." On this side of the pond, there were no few voices suggesting that it was time to retire NATO. Its purpose as a coalition had been served.
And then NATO collectively chose to do out of area ops.
This was not merely "collective defense." It became something new. (ISAF is but one NATO out of area op, but it's one of the longest running ones; the Bosnia IFOR/SFOR mission went for a good while as well)
NATO in the year 2019 is a new thing, though I guess the internal bickering is the same old-same old, but with more voices.
If it is time for any nation to leave NATO, all they have to do is give the rest one year's notice. That's in the treaty.
The question is, how do you throw one out? (Asked with the idea that the Caliphate reestablishes itself in Istanbul - Mr Erdogan now and again gives that impression).
That isn't in the treaty.
Then what? Whence NATO in that case?
Bickering about NATO funding balances would seem a lesser concern at that point.