It wouldn't be good for us if Russia comes in a stops ISIS. We would lose even more standing as the world leader after fighting them for so long and not being able to finish them. I don't think many will look at the fact that ISIS is severely hurt and Russia just finished off our work, if they do come in and finish it...they would just see Russia ending it and give them all the credit.
Russia coming into Afghanistan would be different from their last war with them, for two reasons, in my opinion. 1. We've already hurt ISIS, and they would only have to finish them, and Russian troops wouldn't be limited to fighting a PC war like our troops are. 2. This wouldn't be the previous war where they fought the country of Afghanistan and had the US helping Afghanistan oppose them. Putin could roll in and finish ISIS off fairly quickly, take the glory for Russia, and make Russia an even bigger player on the world stage...which is what he wants anyway. Oh, and to one-up the U.S.