I am happy to see this event is in the game. Russai was, and is, vast, and the people had an almost religious belief in the Tsar and the holiness of Mother Russia. The Poles took Moscow in the early sixteen hundreds, but could not control the nation, as most Russians would not surrender, but simply kept fighting .
If Karl had taken Moscow, hw probably would have faced similar problems, especially if Peter was still Tsar. Peter was a very able strategist, and he did defeat Karl at Poltava, and conquered all of the Swedish empire he could reach by land.