Everything was based on USSR. Had this country allied with Britain & France in 1939, then Germany would have faced an attack on the West. All what the Allies wanted, was to share the casualties with the russians (of course more for the russians...).
If the Allies had attacked in September 1939 (the Saar mini-offensive aside), it would have accelerated their fall. After 10 days, the fate of Poland was certain. The time that France set up an offensive, these 10 days were spent.
The incomplete Siegfried line would have been enough to hold the Allies for 3 or 4 weeks, enough to let the Germans move their troops to the West.
With a long conflict right by 1939, the Allies would have been in a bad shape, because their industry was not already fully mobilized. In 1939, France had 2000 tanks. In May 1940, it had 3500 of them. Keep this in mind.
Poland paid the price of being between two hegemonic powers, Germany and USSR. Unfortunately, it didn't choose the lesser of the two evils by that time, by dating with USSR. A french-british-polish-romanian-russian alliance would have certainly more profitable to Poland. The Allies would have given their warranty to the Polish integrity (as according to the secret negociations, they only planned to shut their mouth when Stalin would invade the baltic states), and it would have probably fixed the Dantzig problem once for all by annexing the german enclave.