I see this argument a lot, and I think it's totally bunk.
The point of military spending isn't to sacrifice a certain number of goats at the altar to ensure the gods' favor, it's to acquire the means to enforce a nation's interests. In our highly industrial age, that means all sorts of ships, submarines, aircraft, launchers and spacecraft, armed and armored vehicles, autonomous {air, ground, sea, undersea} platforms, all sorts of munitions, deep magazines, production lines, domestic supply chains, etc. etc. etc.
The US has spent 3% - 5% of its GDP on its military since 1990, and the US still enjoys the benefits of much of that accumulated spending. Five Nimitz-class aircraft carriers were built even before 1990, when the US was spending 5% - 7% of its GDP on its military. The US still operates B-52Hs, which were built in the 1960's. Even beyond ships and airframes, continued funding of programs and capabilities sustains a sort of inertia of know-how and industrial capability that, once stopped, is difficult and costly to get going again.
Just comparing military spending at a snapshot in time isn't a good way to compare military capabilities and potential. If European nations wish to replace what the US brings to the table, it's going to take a crash rearmament program and very high military spending (easily 10%+ of GDP) for a decade or more. And also a unified command structure, unified procurement, and ultimately probably proper federalization. All of which are, unfortunately, pipe dreams.
