What ultimately ends up happening if you make Leningrad a fortress, is the German player can simply take all the 2 IPC territories in the south, getting the same bonus for Stalingrad, getting the Caucasus bonus, and opening up the middle east.
If you never move those units off of Leningrad, and if Germany decides to make a beeline for Moscow (even as late as a I3/G4 DOW) through Eastern Poland, Western Ukraine, and Bryansk, he can get 20 cheap walking units built on Berlin on G1 and G2 (10I+10Arty) to attack Moscow on G7. And he can back those up with motorized units built on G3 and G4 on Berlin. And with the Bulgarian infantry. And with air built on Western Germany or Berlin on G5, armor built on Ukraine on G6, and bombers built on Western Germany or Berlin on G6.
Yes, you may get an extra 5 IPC for taking Finland on R5. Or 5 on R4 and 11 on R5 if he does an I2/G3 instead of an I3/G4. So you get 5 extra infantry for the defense of Moscow at the price of, what, the 20 pieces you left in Leningrad?
Since Germany has a) long supply line issues and b) the threat of western europe landings, he has to a) avoid battles where he’s trading pieces with the Soviets and b) travel eastward only as slowly as his western defenses will permit. If you’ve got a huge stack on Leningrad, he’s just going to blow by it and pick it up when you can’t build anything else there because he’s got your capital.
That’s why people will do a strategic pull back from all forward locations, including Leningrad. If you can make Germany wait until G8, you’ve got your 20 pieces from the Far East back in Moscow and your stack is around 100 pieces strong. That’s a tough nut for him to crack without dedicating more pieces and more expensive pieces that, if your allies are doing their job, are desperately needed to defend the extensive coastline of Europe and Africa.