Agree, but I think we can also agree that today's age demands more flexible agreements and the ability to make adjustments. That is one of the big reasons why the UK will exit the EU (due to the inflexibility of many EU institutions). I think a 2% of GDP budget for defense is way too high for Central Europe. They are not surrounded my rouge states and hiring top IT/CS talent to combat today's advanced "weaponry" does not cost 2% of GDP, well, maybe Greek's GDP but certainly not Germany's.
I agree with everything you say here. But it should be negotiated through the mechanisms that exist. Simply not paying gives leaders like Trump an excuse to say "They don't pay their fair share".