There is no strategic threat to keep the old lady alive, but deep in their hearts, the Europeans dread the renationalization of their defense policies that would normally follow from victory. The sulky French, always railing against what they see as American Diktat, should understand this. If they ever succeed in cutting the U.S. down to their own size, they will not end up with the leadership of Europe, not even in tandem with the Germans. They will inherit a NATO that will no longer be NATO, but a loose collection of nation states going every which way in matters military. French interests will be better served inside the tent.
History still says no.
The safer the Europeans feel, the less of a premium they will want to pay and the less America will want to care. The more democratic and stable Russia becomes, the stronger its arguments for coming aboard.