Trump seems to think paying a percentage of GDP to NATO is the main concern for members and that the USA is getting short changed by its allies who don't pay up. The USA has mounted an ongoing imperialist project to spread its influence globally, using soft and hard power. This is crucial to its status as the world's only remaining superpower. When countries (like the UK) join NATO, they enter into the Status of Forces Agreement. This allows the US to build bases on sovereign soil and operate militarily within the host state. In effect, the USA gets to plant its flag in every NATO member's territory and maintain a small but powerful slice of US military presence there. They currently have 800 bases worldwide and at least thirteen bases in the UK, which are officially designated as Royal Air Force Bases. They're not RAF bases, they're top secret American military territory. So, by ditching NATO, Trump does indeed leave America's allies to fend for themselves (as if that in itself is good for American global security) but he also jeopardises 800 aircraft carrier-countries that currently host US military might and which enable the USA to continue its global Imperialist Project.
Be careful what you wish for.