While the USSR could definitely defeat the European Axis wholly on its own (Germany, Italy, Finland and the collective forces of Western Europe they amassed). It would be very exhausting, and while the Soviets maintained a large military force in the East to ward off the Japanese, the best they could do without the USA backing them up is push the Japs out of Manchuria and establish control of Northern Continental Asia.
>If Most of East Asia went with the Soviets
I mean... they did. Outside of Japan, whose loss at Khalkin Gol was partly why they took their chances striking at Pearl Harbor, Most of Asia was for the USSR. South Korea and its policies were established by brutal US enforcement and war. SEA had it's communists crushed by US and British forces or hired mercs who brutalized populations and enforced anti-communist criminal laws.
If the USSR had gotten control of East Asia more wholly, the USA would have had a lot less power projection and a lot less capability, while the USSR would have a valuable ally in a whole and functioning DPRK.