US is not doing any defense charity. It's present in Europe only because US commanders think it's in the best interest of US. Europe spends on its military half of what US does.

I don't believe that Europe would spend significantly more on military. Europeans after carrying two world wars on their soil seem little bit resilient to threats of war and to actual violence. When it comes to Americans it seems that goat fart spontaneously combusting on the other side of the planet constitutes experimental discharge of pyrochemical weapon potentially of mass destruction that ought to be carefully monitored by swiftly dispatched fleet of surveillance drones.