In this timeline scenario if both Fance and UK leave the NATO command structure then they are effectively out of NATO. At least in the minds of American planners. France was allowed to have it's cake and eat it too in terms of having a defense industry but outsourcing most ( I know not all) the boots on the ground to the US as it was perceived that that they contributed little. In American public opinion, the UK leaving would have been considered a death blow to the alliance due to remaining mythology in the US regarding British forces. An American first policy with extra sauce for the Monroe doctrine in 1950-1960s possibly leading to direct intervention in Cuba but perhaps not in Vietnam.France never left NATO.