Not to support Trump here, but taken the other way, look at how much value and effectiveness America is bringing to the NATO table. And the cost of having access to that power was a mere 2% of GDP (something that many EU countries that were incredibly reluctant to meet are now increasing frantically to make up for lost time).
it was a perfectly valid argument (and Trump made it into an idiotic talking point in his typical dumbfuck aggressive tantrum style)
the important aspect to keep in mind with other NATO members slacking on their own defense is that it directly increases US influence over them. basically the US and those other members semi-knowingly traded dependence and a to a certain degree subservience for protection.
I don't see how it directly increases US influence over them? America is compelled to defend those NATO members, no matter how rag-tag or inadequate those NATO members' forces are. America doesn't have the option of saying "no", and they also cop all of the public backlash as these countries get to stir up anti-American sentiments and make life hard for American forces that are rotated through these countries for training.
defending someone is not a binary thing. the amount of effort, the degree of severity, the number of troops mobilized, the whole question of use of nukes (tactical or strategic)
and there are real problems around bases (~18-20 year olds on leave are going to do what they are usually do anyway)
and the last few decades of US interventionism is not exactly a success story. it was not hard to stir up "anti-imperialist" sentiment
