The plan was sound in that Moltke hoped for a decisive battle in the Ardennes. The pursuit phase is where the logistics got shakey. We've discussed it before - Moltke's mistake was gambling on the Marne, that's when he went from in control of the situation to not in control of the situation. He should have been content to take Amiens and dig in.
Elements of the British cabinet were a bit like kids just after being told the family was moving to another state. The family is moving, but the process of acceptance takes a while.
I must admit that after watching "The Darkest Hour", I realized how the 'War Cabinet' seemed more of a threat to Churchill than Hitler was. Even Roosevelt abandoned him and the British.