Ok, im just going to say what i think,

While withdrawing back to Poland is not such a bad idea it also has its problems. For the german army to go and take so much in such little time and just pick up and give it back to the Russians would mean total collapse in moral.

Not to mention just straight out leaving could give the Russians the oppertunity to follow the Germans and start harrasing there rear, Which could prevent them from making a defence of Poland.

And then it means abandoning the Baltic states, Which hatted the Russians alot and also abandoning the Ukrain which had Hitler not said "NO" could have supplied 1,000,000 willing soilders.

The only possibilty they would have would be a fighting retreat over a 6 month period which also give time to build defences at key areas.