We will never know, but creating a Polish Kingdom would create several new problems

- First, a king would have to be found. Giving Poland to the King of Saxony would probably make him too powerful for Napoleon's taste, and offend the Poles. A Polish King would offend the Saxons and probably cause a lot of internecine troubles amongst the Poles themselves. A French King (Lucian, perhaps? The King of Rome or Napoleon himself?) could combine the worst aspects of the other solutions and decide Alexander to go to war (he could hope to attract to him a Polish or Saxon King of Poland, but a French?)

- If Saxony lost Poland, the Saxons would probably have to be compensated somehow for their loss, and that wouldn't be easy without stepping on a lot of toes

- Anyway, the new Kingdom of Poland would hurt so seriously Russian, Prussian and Austrian interests that stablishing such a Kingdom wasn't really in France's interest either...

- And, last but not least, Napoleon probably expected victory over Russia to be a relief for his own economic problems