That would be just as dumb. They know what happened in Afghanistan. They know there’s no reason it wouldn’t happen again. If Iranian support of the terrorists in Iraq ended because Russia had taken over we’d have a base of operation on both sides of Iran, and we’ll play the mujahadeen game on them again and in ten years they’ll have to run away.
The Russians will be welcomed by the Iranian leadership, because Iran needs help defending itself from the US presence in Iraq and Afghanistan. When and if the Russians made their move into Iran, attacks against Iraq from Iran would increase, not decrease. The Germans went into Italy to help fortify it because the Italians were not performing well for the axis. If Germany would have prevailed, Italy would have eventually become part of the Third Reich. If Russia prevails, Iran will eventually become part of the Russian Federation. IMHO.