The reason Russia and the UK were determined not to lose Leningrad and Calcutta respectively was the risk those loses would force a negotiated peace.
The loss of Leningrad would severe internal political repercussions in Russia. The loss of India would devastate the “Empire” of the UK.
On the other hand, there was no guarantee that losing those cities would have knocked either country out of the war.
A house rule I will propose for our next FTF is a “Minor Victory” is only final if it is achieved for three turns in a row. A “Moderate Victory” requires two turns in a row. A “Major Victory” is one turn.
This has the effect of not allowing Germany and Japan to ignore everything and go for the Minor Victory by Turn 2 since holding it for three turns is not trivial. On the other hand, the Allies can’t just let those cities fall and not make gains elsewhere or come back to them soon.
This also tends to keep the unit count low. If you don’t have all the time in the world to build up the perfect invasion force / counter attack you tend to fight the war with the military you have. That means low unit count battles…
This also tends to make Africa more of an economic and military sideshow since there are no victory cities there to help with the time pressure of an expansionist Axis. On the other hand, ignoring Africa now puts three infantry into German that actually matter in those low unit count battles.