If IC’s were limited, then abandoning Karelia may indeed be a viable strat (especially if SBR’s were limitted to IPC value of the territory as well).
But, without IC restrictions, the only difference between Russia standing her ground in Karelia and standing her ground in Moscow is that… if they fail in Karelia they have ONE MORE SHOT at holding against Germany, with a UK and possibly a US counter-attack in Karelia to protect them in the interim.