The US continued to keep NATO around, despite the end of the Cold War in the '90s, because it's a huge program for the American defense industry. NATO membership requires a country to have capabilities with certain standards, which largely force them to buy American weaponry. As such, they would never consider full membership for an army-less country, except maybe in very exceptional circumstances (the Vatican? Lol).

But I agree that in the modern world, conventional armies tend to be a massive waste of money. It's just that occasionally a Putin-level threat comes along, and at that point you'll be glad you have them.