One of the great ironies of WW2 is that in 1939, Stalin believed France & Britain were not serious about fighting Germany themselves, and only wanted him as an ally to defeat Germany at the cost of copious amounts of Russian blood. So Stalin, the man who trusted no one, trusted the one man no one should have trusted: Adolf Hitler. He cut his deal with the Non-Aggression Pact. After Hitler dispatched the French, he turned on the USSR. Thus, Stalin wound up doing that which he had tried to avoid; the defeat of Nazi Germany at the cost of copious amounts of Russian blood.
Yes, thanks to the purges and the brutal system of command, the Red Army was club rather than a rapier. But that’s probably what it was designed to be. While they gained tactical and operational competency from 1944 on, the Germans still inflicted immense casualties on the Soviets. And keep in mind, the Germans never had less than 65% of the Wehrmacht on the Eastern Front. Forces we didn’t have to face because of the participation of the USSR.
France falling so quickly, turned out to be a blessing in disguise. Stalin still needed a few more years, most say until 1943, before he would have had the capability to launch an offensive war. By then he was hoping the French, German and British armies were so depleted, so as to enable the Red Army to swoop all the way across Europe. So at least by having France fall, and saving the German and British Armies for the most part, it allowed Hitler to attack Russia, and the Allies to at least win half of Europe and keep it out of the hands of Stalin.