EU is not a unified entity in terms of defense. Thought they might try to stay away from a close tie with US under current administration, I doubt that they can or want to get rid as an ally as whole.
So while I'm not particular pro-brexit but I do think this could have also a positive impact to the EU in regards of more unification or more EU collaborations.
Hell, even Germany are rolling their eyes at a lot of Macron's proposals.