Anyway, Leningrad as a political, industrial and military center was much more important than few almost inhabited regions of Karelia.
Even in our history blockade of Leningrad cost at least 2 million of Soviet lives in Leningrad itself and unknown number on other fronts, which didn't get Leningrad's military production.
Let's play the game - you can change the past:
1) You can totally win the Winter War. Soviet Finland bravely fight against the Germans, there is no blockade of Leningrad, at least two millions of Soviet citizens are saved, Leningrad's production - weapons, tanks, munitions, etc, help to end the war against Germany earlier (say, in August 1944), and against Japan - in April 1945 (preventing all those unnecessary American and Japan deaths).
2) You can prevent the Winter War. In this case German and Finnish forces capture Leningrad and totally destroy it in September 1941. It costs Soviets 3 million lives, war against Germany ended in August 1945, against Japan - in March 1946 with millions of unnecessary death of Japans and Americans.
What would you choose?