Why the Soviets could not simply drive west after Berlin

If the Allies invaded Japan, could the Soviet Union seize the rest of Europe while Allied troops moved east?

No; Soviet logistics were exhausted at Berlin, while strong Allied formations and potential local rearmament remained in Europe.

The Soviet advance reached Berlin with its logistics already near collapse, leaving little fuel, vehicle support, or capacity for another major drive. The Western Allies also kept capable formations in Europe and could rearm German and Italian troops if necessary, so transferring forces toward Japan did not leave the continent undefended.