One problem. Poland would then have to reclaim the land the Soviets stole from them after the war. I don't think the Ukrainians and Belarussians would much care for that.
But it would cure most of the current conflict within the Ukraine between the Catholics and the Orthodox, the Europhiles and those who look toward Russia.
Putin sure would not care for it. And don't forget that Poland used the lack of an effective government in Russia afer WWI to push its Eastern borders well into lands that were primarily populated by non-Poles. Lithuanians, White Russians and Ukrainians. Poland would have a lot more people, but a number of millions of those people would not be, ethnically, Polish.