The French forces (let alone the British) would have had a very tough time attacking across the Franco-German frontier in 1939, the Germans had defences on their side of the border too and realistically speaking the Polish didn't need all of the luftwaffe to defeat them. The Anglo-French forces would have been at serious risk of taking heavy losses even with a great pre-ponderance in forces just in time for the victorious German forces in Poland to return to the western front and smash into them, which leads us to...
The Molotov Ribbentrop pact wasn't just about Germany not having to face Russian forces in taking over Poland it was about being able to then divert most of those same forces to the west to fight the French and British afterwards instead of having to park on the new German-Soviet frontier. Before the pact the Western Allies probably thought that they had time, the pact was announced and concluded ONE WEEK before the invasion of Poland.
Now there is another way to get around the matter of German border defences and guard divisions along the Franco-German border... violate Belgian and possibly even Dutch neutrality and attack Germany that way. Which would mean declaring war on Belgium and the Netherlands which would likely result in them joining Germany
So as I see it the phony war can be laid at the feet of Belgium's ridiculous wishful thinking desire for neutrality and the intelligence agencies of Britain and France who hadn't seen the Molotov Ribbentrop pact coming.