A somewhat more plausible variation would be Stalin killed/toppled in a Coup before WW2.
The whole point of the purges was to prevent any possibility of that happening. The only time Stalin really feared the real possibility of a coup was in early July, 1941. After the opening catastrophes of Operation Barbarossa, he retreated to his dacha and went incommunicado for a few days. Molotov headed up a group of Politburo members to go talk to him. When they entered the dacha, Stalin had an apprehensive look that he expected they were there to arrest him. Instead, they were there to grovel and plead that he come out of hiding and lead his country.
There was no other time before or after when a coup could have happened. The purges were so effective that the only time the opportunity arose, Stalin had so cowed the eunuchs around him that the thought of a coup never occurred to them.