I agree, NATO countries have been riding the apron strings of the US for a long time and need to step up their game and start spending their allotted GDP...my own country included...they have allowed our military to almost become ineffective by not putting any resources into it's development.That was also part of his posturing. He felt like despite our being the biggest payer with the most assets, he got little support from members. This wasn't new to his administration. He just had had enough of us dumping $$ into the world for diminishing returns.
I was in the EU in the 90s. More specifically Hungary and the Czechs.
The efforts leading up to their inclusion was how much $$ would we pour into airports, infrastructure etc. Just to be able to use them strategically. Many of the leaders were Russian holdouts. They became very rich joining our side. We have yet to see any of their assets be put to use for us. But they have world class airports now.
Pulling out of NATO would have still been a disaster....
It seemed that everywhere I deployed (and there were a lot of places) I ran into Americans (military or civilian) who where there for some purpose-mainly to do with resources . It was and still is in the best interest of the US to have a united band of countries standing behind them even if they are a small insignificant country, they all have some sort of trading strength.
If the US were to go it on it's own, all the little dogs would be out biting at it's heels.