And this is their exact problem today- in many respects the entire EU is free riding off the US (and has been for decades now). Within the continent the Spainards (the furthest from Russia with limited world war exposure in their history) have little desire to fund a national defense whereas on the other side you have a Poland (sits on Putin's border and has a long history of being run over from both the east and the west) with the polar opposite view.

But that problem is addressed by having tightly integrated EU defense forces (via NATO). Loosening defense coordination makes that problem worse, not better.

The US did provide a very large proportion of the defense "umbrella" over Europe, but it's not entirely accurate to call that "free riding" - because we got some very important things out of it. Primarily, we got to have an outsized voice in European (and indeed global) security decisions, letting us call the shots on a host of matters and have an influential voice on others. And secondarily, we got the Pax Americana - no wars in Europe for a good 80 years or so. Not bad, considering that European wars cost Americans a lot more lives (and treasure) than any foreign attack against us. There's a very good argument that we're vastly better off paying for Europe's continental defense, since it keeps them weak and us strong and in control of the continent.