As the title says. The challenge is to keep the USSR Stalinist for at least a few decades following Stalin's death. What would happen?
If you somehow arrange circumstances so Molotov is Stalin's successor the USSR will remain Stalinist for some time as he did not die until 1986. Though really, if you avoid Kruschev and his secret speech, The USSR might remain Stalinist longer than it did, depending on who takes Kruschev's place.
It falls apart far sooner than it would have ITTL. Stalinism is exactly as poor a system of running a government as everyone makes it out to be and it gives all other forms of socialism a bad name by existing.