In fact, Trump might leave NATO for this very reason since he keeps complaining that Europeans aren't paying their fair share.

He also did that last time he was in office, which successfully pressured other members into spending more on their militaries. Republicans always treat their allies overseas like shit, and it works. Afterwards, the next Democratic president picks up the slack by making the Euros believe Amerika has returned to being friends with them again. Same dynamic with Dubya and Obama. It's one of the core parts of how well it works for the US to be a one-party state with two ruling parties.