1920s you mean. By the 30s I think the only other old bolshevik leaders still around were Lazar Kaganovich and Kalinin. Maybe Molotov if you count him.

Edit: I tell a lie. Bubov was still in government until 36

Synthesis: 1930s' Soviet Union had a ton of great men, but the material conditions lead to one of them, who was the most fitting for the situation, becoming the general secretary.

I dunno this still smells like idealist thinking to me, it implies the material conditions have some metaphysical power to guide the right people to the right place. Considering how chaotic internal party politics were at the time Lenin could have easily been replaced by Trotsky or some other dildo like that. There could have also been a better person than Stalin for the job we can't really know for sure. Also I'd wager the USSR would maybe still be standing today if Lenin had lived longer than he had and planned the transition of power better.