The issue is that without the US there wouldn't be NATO, and Europe wouldn't have replaced it with anything effective. The closest attempt at an equivalent is the EU army which has never got off the ground, and if it did would be structured in ways optimal for the EU's political goals rather than what makes sense militarily. Think dozens of smaller forces that don't speak each other's languages very well. US DoD has similar issues with congressional pork but it's still one organization and one military under one government.