European nations might buy a lot of hardware from the States, but the US also maintains a large and very expensive military presence there, and everywhere.
So if military relations cool down, the US pulls out of Europe, and Europe exclusively buys its own arms, it might actually save the US money.
But that seems a little far-fetched. There's a lot of transatlantic banter on the internet but when the chips are down, the West will likely pull together yet again.